Method and apparatus for encoding/decoding video using maximum size limitation of chroma transform block, and method for transmitting bitstream

ABSTRACT

An image encoding/decoding method and apparatus are provided. An image decoding method performed by an image decoding apparatus may include determining a prediction mode of a current block, generating a prediction block of the current block based on inter prediction mode information, based on the prediction mode of the current block being an inter prediction mode, generating a residual block of the current block based on a transform block of the current block, and reconstructing the current block based on the prediction block and the residual block of the current bloc.

This application is a Continuation Application of InternationalApplication No. PCT/KR2020/008233, filed on Jun. 24, 2020, which claimsthe benefit of U.S. Provisional Application No. 62/865,951, filed onJun. 24, 2019, the contents of which are all hereby incorporated byreference herein in their entirety.

TECHNICAL FIELD

The present disclosure relates to an image encoding/decoding method andapparatus, and, more particularly, to a method and apparatus forencoding/decoding an image by limiting a maximum size of a chromatransform block, and a method of transmitting a bitstream generated bythe image encoding method/apparatus of the present disclosure.

BACKGROUND ART

Recently, demand for high-resolution and high-quality images such ashigh definition (HD) images and ultra high definition (UHD) images isincreasing in various fields. As resolution and quality of image dataare improved, the amount of transmitted information or bits relativelyincreases as compared to existing image data. An increase in the amountof transmitted information or bits causes an increase in transmissioncost and storage cost.

Accordingly, there is a need for high-efficient image compressiontechnology for effectively transmitting, storing and reproducinginformation on high-resolution and high-quality images.

DISCLOSURE Technical Problem

An object of the present disclosure is to provide an imageencoding/decoding method and apparatus with improved encoding/decodingefficiency.

An object of the present disclosure is to provide an imageencoding/decoding method and apparatus capable of improvingencoding/decoding efficiency by limiting a maximum size of a chromatransform block.

Another object of the present disclosure is to provide a method oftransmitting a bitstream generated by an image encoding method orapparatus according to the present disclosure.

Another object of the present disclosure is to provide a recordingmedium storing a bitstream generated by an image encoding method orapparatus according to the present disclosure.

Another object of the present disclosure is to provide a recordingmedium storing a bitstream received, decoded and used to reconstruct animage by an image decoding apparatus according to the presentdisclosure.

The technical problems solved by the present disclosure are not limitedto the above technical problems and other technical problems which arenot described herein will become apparent to those skilled in the artfrom the following description.

Technical Solution

An image decoding method performed by an image decoding apparatusaccording to an aspect of the present disclosure may include determininga prediction mode of a current block, generating a prediction block ofthe current block based on inter prediction mode information, based onthe prediction mode of the current block being an inter prediction mode,generating a residual block of the current block based on a transformblock of the current block, and reconstructing the current block basedon the prediction block and the residual block of the current block. Inthis case, a size of the transform block may be determined based on acolor component of the current block.

An image decoding apparatus according to an aspect of the presentdisclosure may include a memory and at least one processor. The at leastone processor may determine a prediction mode of a current block,generate a prediction block of the current block based on interprediction mode information, based on the prediction mode of the currentblock being an inter prediction mode, generate a residual block of thecurrent block based on a transform block of the current block, andreconstruct the current block based on the prediction block and theresidual block of the current block. In this case, a size of thetransform block may be determined based on a color component of thecurrent block.

An image encoding method performed by an image encoding apparatusaccording to an aspect of the present disclosure may include determininga current block by splitting an image, generating an inter predictionblock of the current block, generating a residual block of the currentblock based on the inter prediction block, and encoding inter predictionmode information of the current block. In this case, the residual blockmay be encoded based on a size of a transform block of the currentblock, and the size of the transform block may be determined based on acolor component of the current block.

In addition, a transmission method according to another aspect of thepresent disclosure may transmit a bitstream generated by the imageencoding apparatus or the image encoding method of the presentdisclosure.

In addition, a computer-readable recording medium according to anotheraspect of the present disclosure may store the bitstream generated bythe image encoding apparatus or the image encoding method of the presentdisclosure.

The features briefly summarized above with respect to the presentdisclosure are merely exemplary aspects of the detailed descriptionbelow of the present disclosure, and do not limit the scope of thepresent disclosure.

Advantageous Effects

According to the present disclosure, it is possible to provide an imageencoding/decoding method and apparatus with improved encoding/decodingefficiency.

According to the present disclosure, it is possible to provide an imageencoding/decoding method and apparatus capable of improvingencoding/decoding efficiency by limiting a maximum size of a chromatransform block.

Also, according to the present disclosure, it is possible to provide amethod of transmitting a bitstream generated by an image encoding methodor apparatus according to the present disclosure.

Also, according to the present disclosure, it is possible to provide arecording medium storing a bitstream generated by an image encodingmethod or apparatus according to the present disclosure.

Also, according to the present disclosure, it is possible to provide arecording medium storing a bitstream received, decoded and used toreconstruct an image by an image decoding apparatus according to thepresent disclosure.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present disclosure are notlimited to what has been particularly described hereinabove and otheradvantages of the present disclosure will be more clearly understoodfrom the detailed description.

DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically showing a video coding system, to whichan embodiment of the present disclosure is applicable.

FIG. 2 is a view schematically showing an image encoding apparatus, towhich an embodiment of the present disclosure is applicable.

FIG. 3 is a view schematically showing an image decoding apparatus, towhich an embodiment of the present disclosure is applicable.

FIG. 4 is a view showing a partitioning structure of an image accordingto an embodiment.

FIG. 5 is a view showing an embodiment of a partitioning type of a blockaccording to a multi-type tree structure.

FIG. 6 is a view showing a signaling mechanism of block splittinginformation in a quadtree with nested multi-type tree structureaccording to the present disclosure.

FIG. 7 is a view showing an embodiment in which a CTU is partitionedinto multiple CUs.

FIG. 8 is a view illustrating an embodiment of a redundant splittingpattern.

FIG. 9 is a flowchart illustrating an inter prediction based video/imageencoding method.

FIG. 10 is a view illustrating the configuration of an inter predictionunit 180 according to the present disclosure.

FIG. 11 is a flowchart illustrating an inter prediction basedvideo/image decoding method.

FIG. 12 is a view illustrating the configuration of an inter predictionunit 260 according to the present disclosure.

FIG. 13 is a view illustrating neighboring blocks available as a spatialmerge candidate according to an embodiment.

FIG. 14 is a view schematically illustrating a merge candidate listconstruction method according to an embodiment.

FIG. 15 is a view schematically illustrating a motion vector predictorcandidate list construction method according to an embodiment.

FIG. 16 is a view illustrating a syntax structure for transmitting MVDfrom an image encoding apparatus to an image decoding apparatusaccording to an embodiment.

FIG. 17 is a flowchart illustrating an IBC based video/image encodingmethod according to an embodiment.

FIG. 18 is a view illustrating the configuration of a prediction unitfor performing an IBC based video/image encoding method according to anembodiment.

FIG. 19 is a flowchart illustrating an IBC based video/image decodingmethod according to an embodiment.

FIG. 20 is a view illustrating a configuration of a prediction unit forperforming an IBC based video/image decoding method according to anembodiment.

FIG. 21 is a view illustrating syntax for chroma format signalingaccording to an embodiment.

FIG. 22 is a view illustrating a chroma format classification tableaccording to an embodiment.

FIG. 23 is a view illustrating a splitting limitation example of a CUfor virtual pipeline processing.

FIGS. 24 to 26 are views illustrating a splitting example of a CU and aTU according to an embodiment.

FIGS. 27 and 28 are flowcharts illustrating IBC prediction and intraprediction to which a maximum transform size according to an embodimentapplies.

FIG. 29 is a flowchart illustrating a method of encoding an image by anencoding apparatus according to an embodiment.

FIG. 30 is a flowchart illustrating a method of decoding an image by adecoding apparatus according to an embodiment.

FIG. 31 is a view showing a content streaming system, to which anembodiment of the present disclosure is applicable.

MODE FOR INVENTION

Hereinafter, the embodiments of the present disclosure will be describedin detail with reference to the accompanying drawings so as to be easilyimplemented by those skilled in the art. However, the present disclosuremay be implemented in various different forms, and is not limited to theembodiments described herein.

In describing the present disclosure, if it is determined that thedetailed description of a related known function or construction rendersthe scope of the present disclosure unnecessarily ambiguous, thedetailed description thereof will be omitted. In the drawings, parts notrelated to the description of the present disclosure are omitted, andsimilar reference numerals are attached to similar parts.

In the present disclosure, when a component is “connected”, “coupled” or“linked” to another component, it may include not only a directconnection relationship but also an indirect connection relationship inwhich an intervening component is present. In addition, when a component“includes” or “has” other components, it means that other components maybe further included, rather than excluding other components unlessotherwise stated.

In the present disclosure, the terms first, second, etc. may be usedonly for the purpose of distinguishing one component from othercomponents, and do not limit the order or importance of the componentsunless otherwise stated. Accordingly, within the scope of the presentdisclosure, a first component in one embodiment may be referred to as asecond component in another embodiment, and similarly, a secondcomponent in one embodiment may be referred to as a first component inanother embodiment.

In the present disclosure, components that are distinguished from eachother are intended to clearly describe each feature, and do not meanthat the components are necessarily separated. That is, a plurality ofcomponents may be integrated and implemented in one hardware or softwareunit, or one component may be distributed and implemented in a pluralityof hardware or software units. Therefore, even if not stated otherwise,such embodiments in which the components are integrated or the componentis distributed are also included in the scope of the present disclosure.

In the present disclosure, the components described in variousembodiments do not necessarily mean essential components, and somecomponents may be optional components. Accordingly, an embodimentconsisting of a subset of components described in an embodiment is alsoincluded in the scope of the present disclosure. In addition,embodiments including other components in addition to componentsdescribed in the various embodiments are included in the scope of thepresent disclosure.

The present disclosure relates to encoding and decoding of an image, andterms used in the present disclosure may have a general meaning commonlyused in the technical field, to which the present disclosure belongs,unless newly defined in the present disclosure.

In the present disclosure, a “picture” generally refers to a unitrepresenting one image in a specific time period, and a slice/tile is acoding unit constituting a part of a picture, and one picture may becomposed of one or more slices/tiles. In addition, a slice/tile mayinclude one or more coding tree units (CTUs).

In the present disclosure, a “pixel” or a “pel” may mean a smallest unitconstituting one picture (or image). In addition, “sample” may be usedas a term corresponding to a pixel. A sample may generally represent apixel or a value of a pixel, and may represent only a pixel/pixel valueof a luma component or only a pixel/pixel value of a chroma component.

In the present disclosure, a “unit” may represent a basic unit of imageprocessing. The unit may include at least one of a specific region ofthe picture and information related to the region. The unit may be usedinterchangeably with terms such as “sample array”, “block” or “area” insome cases. In a general case, an M×N block may include samples (orsample arrays) or a set (or array) of transform coefficients of Mcolumns and N rows.

In the present disclosure, “current block” may mean one of “currentcoding block”, “current coding unit”, “coding target block”, “decodingtarget block” or “processing target block”. When prediction isperformed, “current block” may mean “current prediction block” or“prediction target block”. When transform (inversetransform)/quantization (dequantization) is performed, “current block”may mean “current transform block” or “transform target block”. Whenfiltering is performed, “current block” may mean “filtering targetblock”.

In addition, in the present disclosure, a “current block” may mean “aluma block of a current block” unless explicitly stated as a chromablock. The “chroma block of the current block” may be expressed byincluding an explicit description of a chroma block, such as “chromablock” or “current chroma block”.

In the present disclosure, the term “/” and “,” should be interpreted toindicate “and/or.” For instance, the expression “A/B” and “A, B” maymean “A and/or B.” Further, “A/B/C” and “A/B/C” may mean “at least oneof A, B, and/or C.”

In the present disclosure, the term “or” should be interpreted toindicate “and/or.” For instance, the expression “A or B” may comprise 1)only “A”, 2) only “B”, and/or 3) both “A and B”. In other words, in thepresent disclosure, the term “or” should be interpreted to indicate“additionally or alternatively.”

Overview of Video Coding System

FIG. 1 is a view showing a video coding system according to the presentdisclosure.

The video coding system according to an embodiment may include aencoding apparatus 10 and a decoding apparatus 20. The encodingapparatus 10 may deliver encoded video and/or image information or datato the decoding apparatus 20 in the form of a file or streaming via adigital storage medium or network.

The encoding apparatus 10 according to an embodiment may include a videosource generator 11, an encoding unit 12 and a transmitter 13. Thedecoding apparatus 20 according to an embodiment may include a receiver21, a decoding unit 22 and a renderer 23. The encoding unit 12 may becalled a video/image encoding unit, and the decoding unit 22 may becalled a video/image decoding unit. The transmitter 13 may be includedin the encoding unit 12. The receiver 21 may be included in the decodingunit 22. The renderer 23 may include a display and the display may beconfigured as a separate device or an external component.

The video source generator 11 may acquire a video/image through aprocess of capturing, synthesizing or generating the video/image. Thevideo source generator 11 may include a video/image capture deviceand/or a video/image generating device. The video/image capture devicemay include, for example, one or more cameras, video/image archivesincluding previously captured video/images, and the like. Thevideo/image generating device may include, for example, computers,tablets and smartphones, and may (electronically) generate video/images.For example, a virtual video/image may be generated through a computeror the like. In this case, the video/image capturing process may bereplaced by a process of generating related data.

The encoding unit 12 may encode an input video/image. The encoding unit12 may perform a series of procedures such as prediction, transform, andquantization for compression and coding efficiency. The encoding unit 12may output encoded data (encoded video/image information) in the form ofa bitstream.

The transmitter 13 may transmit the encoded video/image information ordata output in the form of a bitstream to the receiver 21 of thedecoding apparatus 20 through a digital storage medium or a network inthe form of a file or streaming. The digital storage medium may includevarious storage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, andthe like. The transmitter 13 may include an element for generating amedia file through a predetermined file format and may include anelement for transmission through a broadcast/communication network. Thereceiver 21 may extract/receive the bitstream from the storage medium ornetwork and transmit the bitstream to the decoding unit 22.

The decoding unit 22 may decode the video/image by performing a seriesof procedures such as dequantization, inverse transform, and predictioncorresponding to the operation of the encoding unit 12.

The renderer 23 may render the decoded video/image. The renderedvideo/image may be displayed through the display.

Overview of Image Encoding Apparatus

FIG. 2 is a view schematically showing an image encoding apparatus, towhich an embodiment of the present disclosure is applicable.

As shown in FIG. 2 , the image encoding apparatus 100 may include animage partitioner 110, a subtractor 115, a transformer 120, a quantizer130, a dequantizer 140, an inverse transformer 150, an adder 155, afilter 160, a memory 170, an inter prediction unit 180, an intraprediction unit 185 and an entropy encoder 190. The inter predictionunit 180 and the intra prediction unit 185 may be collectively referredto as a “prediction unit”. The transformer 120, the quantizer 130, thedequantizer 140 and the inverse transformer 150 may be included in aresidual processor. The residual processor may further include thesubtractor 115.

All or at least some of the plurality of components configuring theimage encoding apparatus 100 may be configured by one hardware component(e.g., an encoder or a processor) in some embodiments. In addition, thememory 170 may include a decoded picture buffer (DPB) and may beconfigured by a digital storage medium.

The image partitioner 110 may partition an input image (or a picture ora frame) input to the image encoding apparatus 100 into one or moreprocessing units. For example, the processing unit may be called acoding unit (CU). The coding unit may be acquired by recursivelypartitioning a coding tree unit (CTU) or a largest coding unit (LCU)according to a quad-tree binary-tree ternary-tree (QT/BT/TT) structure.For example, one coding unit may be partitioned into a plurality ofcoding units of a deeper depth based on a quad tree structure, a binarytree structure, and/or a ternary structure. For partitioning of thecoding unit, a quad tree structure may be applied first and the binarytree structure and/or ternary structure may be applied later. The codingprocedure according to the present disclosure may be performed based onthe final coding unit that is no longer partitioned. The largest codingunit may be used as the final coding unit or the coding unit of deeperdepth acquired by partitioning the largest coding unit may be used asthe final coding unit. Here, the coding procedure may include aprocedure of prediction, transform, and reconstruction, which will bedescribed later. As another example, the processing unit of the codingprocedure may be a prediction unit (PU) or a transform unit (TU). Theprediction unit and the transform unit may be split or partitioned fromthe final coding unit. The prediction unit may be a unit of sampleprediction, and the transform unit may be a unit for deriving atransform coefficient and/or a unit for deriving a residual signal fromthe transform coefficient.

The prediction unit (the inter prediction unit 180 or the intraprediction unit 185) may perform prediction on a block to be processed(current block) and generate a predicted block including predictionsamples for the current block. The prediction unit may determine whetherintra prediction or inter prediction is applied on a current block or CUbasis. The prediction unit may generate various information related toprediction of the current block and transmit the generated informationto the entropy encoder 190. The information on the prediction may beencoded in the entropy encoder 190 and output in the form of abitstream.

The intra prediction unit 185 may predict the current block by referringto the samples in the current picture. The referred samples may belocated in the neighborhood of the current block or may be located apartaccording to the intra prediction mode and/or the intra predictiontechnique. The intra prediction modes may include a plurality ofnon-directional modes and a plurality of directional modes. Thenon-directional mode may include, for example, a DC mode and a planarmode. The directional mode may include, for example, 33 directionalprediction modes or 65 directional prediction modes according to thedegree of detail of the prediction direction. However, this is merely anexample, more or less directional prediction modes may be used dependingon a setting. The intra prediction unit 185 may determine the predictionmode applied to the current block by using a prediction mode applied toa neighboring block.

The inter prediction unit 180 may derive a predicted block for thecurrent block based on a reference block (reference sample array)specified by a motion vector on a reference picture. In this case, inorder to reduce the amount of motion information transmitted in theinter prediction mode, the motion information may be predicted in unitsof blocks, subblocks, or samples based on correlation of motioninformation between the neighboring block and the current block. Themotion information may include a motion vector and a reference pictureindex. The motion information may further include inter predictiondirection (L0 prediction, L1 prediction, Bi prediction, etc.)information. In the case of inter prediction, the neighboring block mayinclude a spatial neighboring block present in the current picture and atemporal neighboring block present in the reference picture. Thereference picture including the reference block and the referencepicture including the temporal neighboring block may be the same ordifferent. The temporal neighboring block may be called a collocatedreference block, a co-located CU (colCU), and the like. The referencepicture including the temporal neighboring block may be called acollocated picture (colPic). For example, the inter prediction unit 180may configure a motion information candidate list based on neighboringblocks and generate information specifying which candidate is used toderive a motion vector and/or a reference picture index of the currentblock. Inter prediction may be performed based on various predictionmodes. For example, in the case of a skip mode and a merge mode, theinter prediction unit 180 may use motion information of the neighboringblock as motion information of the current block. In the case of theskip mode, unlike the merge mode, the residual signal may not betransmitted. In the case of the motion vector prediction (MVP) mode, themotion vector of the neighboring block may be used as a motion vectorpredictor, and the motion vector of the current block may be signaled byencoding a motion vector difference and an indicator for a motion vectorpredictor. The motion vector difference may mean a difference betweenthe motion vector of the current block and the motion vector predictor.

The prediction unit may generate a prediction signal based on variousprediction methods and prediction techniques described below. Forexample, the prediction unit may not only apply intra prediction orinter prediction but also simultaneously apply both intra prediction andinter prediction, in order to predict the current block. A predictionmethod of simultaneously applying both intra prediction and interprediction for prediction of the current block may be called combinedinter and intra prediction (CIIP). In addition, the prediction unit mayperform intra block copy (IBC) for prediction of the current block.Intra block copy may be used for content image/video coding of a game orthe like, for example, screen content coding (SCC). IBC is a method ofpredicting a current picture using a previously reconstructed referenceblock in the current picture at a location apart from the current blockby a predetermined distance. When IBC is applied, the location of thereference block in the current picture may be encoded as a vector (blockvector) corresponding to the predetermined distance. IBC basicallyperforms prediction in the current picture, but may be performedsimilarly to inter prediction in that a reference block is derivedwithin the current picture. That is, IBC may use at least one of theinter prediction techniques described in the present disclosure.

The prediction signal generated by the prediction unit may be used togenerate a reconstructed signal or to generate a residual signal. Thesubtractor 115 may generate a residual signal (residual block orresidual sample array) by subtracting the prediction signal (predictedblock or prediction sample array) output from the prediction unit fromthe input image signal (original block or original sample array). Thegenerated residual signal may be transmitted to the transformer 120.

The transformer 120 may generate transform coefficients by applying atransform technique to the residual signal. For example, the transformtechnique may include at least one of a discrete cosine transform (DCT),a discrete sine transform (DST), a karhunen-loéve transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, the GBT means transform obtained from a graph whenrelationship information between pixels is represented by the graph. TheCNT refers to transform acquired based on a prediction signal generatedusing all previously reconstructed pixels. In addition, the transformprocess may be applied to square pixel blocks having the same size ormay be applied to blocks having a variable size rather than square.

The quantizer 130 may quantize the transform coefficients and transmitthem to the entropy encoder 190. The entropy encoder 190 may encode thequantized signal (information on the quantized transform coefficients)and output a bitstream. The information on the quantized transformcoefficients may be referred to as residual information. The quantizer130 may rearrange quantized transform coefficients in a block form intoa one-dimensional vector form based on a coefficient scanning order andgenerate information on the quantized transform coefficients based onthe quantized transform coefficients in the one-dimensional vector form.

The entropy encoder 190 may perform various encoding methods such as,for example, exponential Golomb, context-adaptive variable length coding(CAVLC), context-adaptive binary arithmetic coding (CABAC), and thelike. The entropy encoder 190 may encode information necessary forvideo/image reconstruction other than quantized transform coefficients(e.g., values of syntax elements, etc.) together or separately. Encodedinformation (e.g., encoded video/image information) may be transmittedor stored in units of network abstraction layers (NALs) in the form of abitstream. The video/image information may further include informationon various parameter sets such as an adaptation parameter set (APS), apicture parameter set (PPS), a sequence parameter set (SPS), or a videoparameter set (VPS). In addition, the video/image information mayfurther include general constraint information. The signaledinformation, transmitted information and/or syntax elements described inthe present disclosure may be encoded through the above-describedencoding procedure and included in the bitstream.

The bitstream may be transmitted over a network or may be stored in adigital storage medium. The network may include a broadcasting networkand/or a communication network, and the digital storage medium mayinclude various storage media such as USB, SD, CD, DVD, Blu-ray, HDD,SSD, and the like. A transmitter (not shown) transmitting a signaloutput from the entropy encoder 190 and/or a storage unit (not shown)storing the signal may be included as internal/external element of theimage encoding apparatus 100. Alternatively, the transmitter may beprovided as the component of the entropy encoder 190.

The quantized transform coefficients output from the quantizer 130 maybe used to generate a residual signal. For example, the residual signal(residual block or residual samples) may be reconstructed by applyingdequantization and inverse transform to the quantized transformcoefficients through the dequantizer 140 and the inverse transformer150.

The adder 155 adds the reconstructed residual signal to the predictionsignal output from the inter prediction unit 180 or the intra predictionunit 185 to generate a reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). If there is noresidual for the block to be processed, such as a case where the skipmode is applied, the predicted block may be used as the reconstructedblock. The adder 155 may be called a reconstructor or a reconstructedblock generator. The generated reconstructed signal may be used forintra prediction of a next block to be processed in the current pictureand may be used for inter prediction of a next picture through filteringas described below.

The filter 160 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter160 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 170, specifically, a DPB of thememory 170. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 160 may generate variousinformation related to filtering and transmit the generated informationto the entropy encoder 190 as described later in the description of eachfiltering method. The information related to filtering may be encoded bythe entropy encoder 190 and output in the form of a bitstream.

The modified reconstructed picture transmitted to the memory 170 may beused as the reference picture in the inter prediction unit 180. Wheninter prediction is applied through the image encoding apparatus 100,prediction mismatch between the image encoding apparatus 100 and theimage decoding apparatus may be avoided and encoding efficiency may beimproved.

The DPB of the memory 170 may store the modified reconstructed picturefor use as a reference picture in the inter prediction unit 180. Thememory 170 may store the motion information of the block from which themotion information in the current picture is derived (or encoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter prediction unit 180 and used as the motion information of thespatial neighboring block or the motion information of the temporalneighboring block. The memory 170 may store reconstructed samples ofreconstructed blocks in the current picture and may transfer thereconstructed samples to the intra prediction unit 185.

Overview of Image Decoding Apparatus

FIG. 3 is a view schematically showing an image decoding apparatus, towhich an embodiment of the present disclosure is applicable.

As shown in FIG. 3 , the image decoding apparatus 200 may include anentropy decoder 210, a dequantizer 220, an inverse transformer 230, anadder 235, a filter 240, a memory 250, an inter prediction unit 260 andan intra prediction unit 265. The inter prediction unit 260 and theintra prediction unit 265 may be collectively referred to as a“prediction unit”. The dequantizer 220 and the inverse transformer 230may be included in a residual processor.

All or at least some of a plurality of components configuring the imagedecoding apparatus 200 may be configured by a hardware component (e.g.,a decoder or a processor) according to an embodiment. In addition, thememory 250 may include a decoded picture buffer (DPB) or may beconfigured by a digital storage medium.

The image decoding apparatus 200, which has received a bitstreamincluding video/image information, may reconstruct an image byperforming a process corresponding to a process performed by the imageencoding apparatus 100 of FIG. 2 . For example, the image decodingapparatus 200 may perform decoding using a processing unit applied inthe image encoding apparatus. Thus, the processing unit of decoding maybe a coding unit, for example. The coding unit may be acquired bypartitioning a coding tree unit or a largest coding unit. Thereconstructed image signal decoded and output through the image decodingapparatus 200 may be reproduced through a reproducing apparatus (notshown).

The image decoding apparatus 200 may receive a signal output from theimage encoding apparatus of FIG. 2 in the form of a bitstream. Thereceived signal may be decoded through the entropy decoder 210. Forexample, the entropy decoder 210 may parse the bitstream to deriveinformation (e.g., video/image information) necessary for imagereconstruction (or picture reconstruction). The video/image informationmay further include information on various parameter sets such as anadaptation parameter set (APS), a picture parameter set (PPS), asequence parameter set (SPS), or a video parameter set (VPS). Inaddition, the video/image information may further include generalconstraint information. The image decoding apparatus may further decodepicture based on the information on the parameter set and/or the generalconstraint information. Signaled/received information and/or syntaxelements described in the present disclosure may be decoded through thedecoding procedure and obtained from the bitstream. For example, theentropy decoder 210 decodes the information in the bitstream based on acoding method such as exponential Golomb coding, CAVLC, or CABAC, andoutput values of syntax elements required for image reconstruction andquantized values of transform coefficients for residual. Morespecifically, the CABAC entropy decoding method may receive a bincorresponding to each syntax element in the bitstream, determine acontext model using a decoding target syntax element information,decoding information of a neighboring block and a decoding target blockor information of a symbol/bin decoded in a previous stage, and performarithmetic decoding on the bin by predicting a probability of occurrenceof a bin according to the determined context model, and generate asymbol corresponding to the value of each syntax element. In this case,the CABAC entropy decoding method may update the context model by usingthe information of the decoded symbol/bin for a context model of a nextsymbol/bin after determining the context model. The information relatedto the prediction among the information decoded by the entropy decoder210 may be provided to the prediction unit (the inter prediction unit260 and the intra prediction unit 265), and the residual value on whichthe entropy decoding was performed in the entropy decoder 210, that is,the quantized transform coefficients and related parameter information,may be input to the dequantizer 220. In addition, information onfiltering among information decoded by the entropy decoder 210 may beprovided to the filter 240. Meanwhile, a receiver (not shown) forreceiving a signal output from the image encoding apparatus may befurther configured as an internal/external element of the image decodingapparatus 200, or the receiver may be a component of the entropy decoder210.

Meanwhile, the image decoding apparatus according to the presentdisclosure may be referred to as a video/image/picture decodingapparatus. The image decoding apparatus may be classified into aninformation decoder (video/image/picture information decoder) and asample decoder (video/image/picture sample decoder). The informationdecoder may include the entropy decoder 210. The sample decoder mayinclude at least one of the dequantizer 220, the inverse transformer230, the adder 235, the filter 240, the memory 250, the inter predictionunit 260 or the intra prediction unit 265.

The dequantizer 220 may dequantize the quantized transform coefficientsand output the transform coefficients. The dequantizer 220 may rearrangethe quantized transform coefficients in the form of a two-dimensionalblock. In this case, the rearrangement may be performed based on thecoefficient scanning order performed in the image encoding apparatus.The dequantizer 220 may perform dequantization on the quantizedtransform coefficients by using a quantization parameter (e.g.,quantization step size information) and obtain transform coefficients.

The inverse transformer 230 may inversely transform the transformcoefficients to obtain a residual signal (residual block, residualsample array).

The prediction unit may perform prediction on the current block andgenerate a predicted block including prediction samples for the currentblock. The prediction unit may determine whether intra prediction orinter prediction is applied to the current block based on theinformation on the prediction output from the entropy decoder 210 andmay determine a specific intra/inter prediction mode (predictiontechnique).

It is the same as described in the prediction unit of the image encodingapparatus 100 that the prediction unit may generate the predictionsignal based on various prediction methods (techniques) which will bedescribed later.

The intra prediction unit 265 may predict the current block by referringto the samples in the current picture. The description of the intraprediction unit 185 is equally applied to the intra prediction unit 265.

The inter prediction unit 260 may derive a predicted block for thecurrent block based on a reference block (reference sample array)specified by a motion vector on a reference picture. In this case, inorder to reduce the amount of motion information transmitted in theinter prediction mode, motion information may be predicted in units ofblocks, subblocks, or samples based on correlation of motion informationbetween the neighboring block and the current block. The motioninformation may include a motion vector and a reference picture index.The motion information may further include inter prediction direction(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. For example, theinter prediction unit 260 may configure a motion information candidatelist based on neighboring blocks and derive a motion vector of thecurrent block and/or a reference picture index based on the receivedcandidate selection information. Inter prediction may be performed basedon various prediction modes, and the information on the prediction mayinclude information specifying a mode of inter prediction for thecurrent block.

The adder 235 may generate a reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) by adding theobtained residual signal to the prediction signal (predicted block,predicted sample array) output from the prediction unit (including theinter prediction unit 260 and/or the intra prediction unit 265). Ifthere is no residual for the block to be processed, such as when theskip mode is applied, the predicted block may be used as thereconstructed block. The description of the adder 155 is equallyapplicable to the adder 235. The adder 235 may be called a reconstructoror a reconstructed block generator. The generated reconstructed signalmay be used for intra prediction of a next block to be processed in thecurrent picture and may be used for inter prediction of a next picturethrough filtering as described below.

The filter 240 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter240 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 250, specifically, a DPB of thememory 250. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 250may be used as a reference picture in the inter prediction unit 260. Thememory 250 may store the motion information of the block from which themotion information in the current picture is derived (or decoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter prediction unit 260 so as to be utilized as the motioninformation of the spatial neighboring block or the motion informationof the temporal neighboring block. The memory 250 may storereconstructed samples of reconstructed blocks in the current picture andtransfer the reconstructed samples to the intra prediction unit 265.

In the present disclosure, the embodiments described in the filter 160,the inter prediction unit 180, and the intra prediction unit 185 of theimage encoding apparatus 100 may be equally or correspondingly appliedto the filter 240, the inter prediction unit 260, and the intraprediction unit 265 of the image decoding apparatus 200.

Overview of Image Partitioning

The video/image coding method according to the present disclosure may beperformed based on an image partitioning structure as follows.Specifically, the procedures of prediction, residual processing((inverse) transform, (de)quantization, etc.), syntax element coding,and filtering, which will be described later, may be performed based ona CTU, CU (and/or TU, PU) derived based on the image partitioningstructure. The image may be partitioned in block units and the blockpartitioning procedure may be performed in the image partitioner 110 ofthe encoding apparatus. The partitioning related information may beencoded by the entropy encoder 190 and transmitted to the decodingapparatus in the form of a bitstream. The entropy decoder 210 of thedecoding apparatus may derive a block partitioning structure of thecurrent picture based on the partitioning related information obtainedfrom the bitstream, and based on this, may perform a series ofprocedures (e.g., prediction, residual processing, block/picturereconstruction, in-loop filtering, etc.) for image decoding.

Pictures may be partitioned into a sequence of coding tree units (CTUs).FIG. 4 shows an example in which a picture is partitioned into CTUs. TheCTU may correspond to a coding tree block (CTB). Alternatively, the CTUmay include a coding tree block of luma samples and two coding treeblocks of corresponding chroma samples. For example, for a picture thatcontains three sample arrays, the CTU may include an N×N block of lumasamples and two corresponding blocks of chroma samples.

Overview of Partitioning of CTU

As described above, the coding unit may be acquired by recursivelypartitioning the coding tree unit (CTU) or the largest coding unit (LCU)according to a quad-tree/binary-tree/ternary-tree (QT/BT/TT) structure.For example, the CTU may be first partitioned into quadtree structures.Thereafter, leaf nodes of the quadtree structure may be furtherpartitioned by a multi-type tree structure.

Partitioning according to quadtree means that a current CU (or CTU) ispartitioned into equally four. By partitioning according to quadtree,the current CU may be partitioned into four CUs having the same widthand the same height. When the current CU is no longer partitioned intothe quadtree structure, the current CU corresponds to the leaf node ofthe quad-tree structure. The CU corresponding to the leaf node of thequadtree structure may be no longer partitioned and may be used as theabove-described final coding unit. Alternatively, the CU correspondingto the leaf node of the quadtree structure may be further partitioned bya multi-type tree structure.

FIG. 5 is a view showing an embodiment of a partitioning type of a blockaccording to a multi-type tree structure. Partitioning according to themulti-type tree structure may include two types of splitting accordingto a binary tree structure and two types of splitting according to aternary tree structure.

The two types of splitting according to the binary tree structure mayinclude vertical binary splitting (SPLIT_BT_VER) and horizontal binarysplitting (SPLIT_BT_HOR). Vertical binary splitting (SPLIT_BT_VER) meansthat the current CU is split into equally two in the vertical direction.As shown in FIG. 4 , by vertical binary splitting, two CUs having thesame height as the current CU and having a width which is half the widthof the current CU may be generated. Horizontal binary splitting(SPLIT_BT_HOR) means that the current CU is split into equally two inthe horizontal direction. As shown in FIG. 5 , by horizontal binarysplitting, two CUs having a height which is half the height of thecurrent CU and having the same width as the current CU may be generated.

Two types of splitting according to the ternary tree structure mayinclude vertical ternary splitting (SPLIT_TT_VER) and horizontal ternarysplitting (SPLIT_TT_HOR). In vertical ternary splitting (SPLIT_TT_VER),the current CU is split in the vertical direction at a ratio of 1:2:1.As shown in FIG. 5 , by vertical ternary splitting, two CUs having thesame height as the current CU and having a width which is ¼ of the widthof the current CU and a CU having the same height as the current CU andhaving a width which is half the width of the current CU may begenerated. In horizontal ternary splitting (SPLIT_TT_HOR), the currentCU is split in the horizontal direction at a ratio of 1:2:1. As shown inFIG. 5 , by horizontal ternary splitting, two CUs having a height whichis ¼ of the height of the current CU and having the same width as thecurrent CU and a CU having a height which is half the height of thecurrent CU and having the same width as the current CU may be generated.

FIG. 6 is a view showing a signaling mechanism of block splittinginformation in a quadtree with nested multi-type tree structureaccording to the present disclosure.

Here, the CTU is treated as the root node of the quadtree, and ispartitioned for the first time into a quadtree structure. Information(e.g., qt_split_flag) specifying whether quadtree splitting is performedon the current CU (CTU or node (QT_node) of the quadtree) is signaled.For example, when qt_split_flag has a first value (e.g., “1”), thecurrent CU may be quadtree-partitioned. In addition, when qt_split_flaghas a second value (e.g., “0”), the current CU is notquadtree-partitioned, but becomes the leaf node (QT_leaf_node) of thequadtree. Each quadtree leaf node may then be further partitioned intomultitype tree structures. That is, the leaf node of the quadtree maybecome the node (MTT_node) of the multi-type tree. In the multitype treestructure, a first flag (e.g., Mtt_split_cu_flag) is signaled to specifywhether the current node is additionally partitioned. If thecorresponding node is additionally partitioned (e.g., if the first flagis 1), a second flag (e.g., Mtt_split_cu_vertical_flag) may be signaledto specify the splitting direction. For example, the splitting directionmay be a vertical direction if the second flag is 1 and may be ahorizontal direction if the second flag is 0. Then, a third flag (e.g.,Mtt_split_cu_binary_flag) may be signaled to specify whether the splittype is a binary split type or a ternary split type. For example, thesplit type may be a binary split type when the third flag is 1 and maybe a ternary split type when the third flag is 0. The node of themulti-type tree acquired by binary splitting or ternary splitting may befurther partitioned into multi-type tree structures. However, the nodeof the multi-type tree may not be partitioned into quadtree structures.If the first flag is 0, the corresponding node of the multi-type tree isno longer split but becomes the leaf node (MTT_leaf_node) of themulti-type tree. The CU corresponding to the leaf node of the multi-typetree may be used as the above-described final coding unit.

Based on the mtt_split_cu_vertical_flag and themtt_split_cu_binary_flag, a multi-type tree splitting mode(MttSplitMode) of a CU may be derived as shown in Table 1 below. In thefollowing description, the multi-type tree splitting mode may bereferred to as a multi-tree splitting type or splitting type.

TABLE 1 MttSplitMode mtt_split_cu_vertical_flag mtt_split_cu_binary_flagSPLIT_TT_HOR 0 0 SPLIT_BT_HOR 0 1 SPLIT_TT_VER 1 0 SPLIT_BT_VER 1 1

FIG. 7 is a view showing an example in which a CTU is partitioned intomultiple CUs by applying a multi-type tree after applying a quadtree. InFIG. 7 , bold block edges 710 represent quadtree partitioning and theremaining edges 720 represent multitype tree partitioning. The CU maycorrespond to a coding block (CB). In an embodiment, the CU may includea coding block of luma samples and two coding blocks of chroma samplescorresponding to the luma samples. A chroma component (sample) CB or TBsize may be derived based on a luma component (sample) CB or TB sizeaccording to the component ratio according to the color format (chromaformat, e.g., 4:4:4, 4:2:2, 4:2:0 or the like) of the picture/image. Incase of 4:4:4 color format, the chroma component CB/TB size may be setequal to be luma component CB/TB size. In case of 4:2:2 color format,the width of the chroma component CB/TB may be set to half the width ofthe luma component CB/TB and the height of the chroma component CB/TBmay be set to the height of the luma component CB/TB. In case of 4:2:0color format, the width of the chroma component CB/TB may be set to halfthe width of the luma component CB/TB and the height of the chromacomponent CB/TB may be set to half the height of the luma componentCB/TB.

In an embodiment, when the size of the CTU is 128 based on the lumasample unit, the size of the CU may have a size from 128×128 to 4×4which is the same size as the CTU. In one embodiment, in case of 4:2:0color format (or chroma format), a chroma CB size may have a size from64×64 to 2×2.

Meanwhile, in an embodiment, the CU size and the TU size may be thesame. Alternatively, there may be a plurality of TUs in a CU region. TheTU size generally represents a luma component (sample) transform block(TB) size.

The TU size may be derived based a largest allowable TB size maxTbSizewhich is a predetermined value. For example, when the CU size is greaterthan maxTbSize, a plurality of TUs (TBs) having maxTbSize may be derivedfrom the CU and transform/inverse transform may be performed in units ofTU (TB). For example, the largest allowable luma TB size may be 64×64and the largest allowable chroma TB size may be 32×32. If the width orheight of the CB partitioned according to the tree structure is largerthan the largest transform width or height, the CB may be automatically(or implicitly) partitioned until the TB size limit in the horizontaland vertical directions is satisfied.

In addition, for example, when intra prediction is applied, an intraprediction mode/type may be derived in units of CU (or CB) and aneighboring reference sample derivation and prediction sample generationprocedure may be performed in units of TU (or TB). In this case, theremay be one or a plurality of TUs (or TBs) in one CU (or CB) region and,in this case, the plurality of TUs or (TBs) may share the same intraprediction mode/type.

Meanwhile, for a quadtree coding tree scheme with nested multitype tree,the following parameters may be signaled as SPS syntax elements from theencoding apparatus to the decoding apparatus. For example, at least oneof a CTU size which is a parameter representing the root node size of aquadtree, MinQTSize which is a parameter representing the minimumallowed quadtree leaf node size, MaxBtSize which is a parameterrepresenting the maximum allowed binary tree root node size, MaxTtSizewhich is a parameter representing the maximum allowed ternary tree rootnode size, MaxMttDepth which is a parameter representing the maximumallowed hierarchy depth of multi-type tree splitting from a quadtreeleaf node, MinBtSize which is a parameter representing the minimumallowed binary tree leaf node size, or MinTtSize which is a parameterrepresenting the minimum allowed ternary tree leaf node size issignaled.

As an embodiment of using 4:2:0 chroma format, the CTU size may be setto 128×128 luma blocks and two 64×64 chroma blocks corresponding to theluma blocks. In this case, MinOTSize may be set to 16×16, MaxBtSize maybe set to 128×128, MaxTtSzie may be set to 64×64, MinBtSize andMinTtSize may be set to 4×4, and MaxMttDepth may be set to 4. Quadtreepartitioning may be applied to the CTU to generate quadtree leaf nodes.The quadtree leaf node may be called a leaf QT node. Quadtree leaf nodesmay have a size from a 16×16 size (e.g., the MinOTSize) to a 128×128size (e.g., the CTU size). If the leaf QT node is 128×128, it may not beadditionally partitioned into a binary tree/ternary tree. This isbecause, in this case, even if partitioned, it exceeds MaxBtsize andMaxTtszie (e.g., 64×64). In other cases, leaf QT nodes may be furtherpartitioned into a multitype tree. Therefore, the leaf QT node is theroot node for the multitype tree, and the leaf QT node may have amultitype tree depth (mttDepth) 0 value. If the multitype tree depthreaches MaxMttdepth (e.g., 4), further partitioning may not beconsidered further. If the width of the multitype tree node is equal toMinBtSize and less than or equal to 2×MinTtSize, then no furtherhorizontal partitioning may be considered. If the height of themultitype tree node is equal to MinBtSize and less than or equal to2×MinTtSize, no further vertical partitioning may be considered. Whenpartitioning is not considered, the encoding apparatus may skipsignaling of partitioning information. In this case, the decodingapparatus may derive partitioning information with a predeterminedvalue.

Meanwhile, one CTU may include a coding block of luma samples(hereinafter referred to as a “luma block”) and two coding blocks ofchroma samples corresponding thereto (hereinafter referred to as “chromablocks”). The above-described coding tree scheme may be equally orseparately applied to the luma block and chroma block of the current CU.Specifically, the luma and chroma blocks in one CTU may be partitionedinto the same block tree structure and, in this case, the tree structureis represented as SINGLE_TREE. Alternatively, the luma and chroma blocksin one CTU may be partitioned into separate block tree structures, and,in this case, the tree structure may be represented as DUAL_TREE. Thatis, when the CTU is partitioned into dual trees, the block treestructure for the luma block and the block tree structure for the chromablock may be separately present. In this case, the block tree structurefor the luma block may be called DUAL_TREE_LUMA, and the block treestructure for the chroma component may be called DUAL_TREE_CHROMA. For Pand B slice/tile groups, luma and chroma blocks in one CTU may belimited to have the same coding tree structure. However, for Islice/tile groups, luma and chroma blocks may have a separate block treestructure from each other. If the separate block tree structure isapplied, the luma CTB may be partitioned into CUs based on a particularcoding tree structure, and the chroma CTB may be partitioned into chromaCUs based on another coding tree structure. That is, this means that aCU in an I slice/tile group, to which the separate block tree structureis applied, may include a coding block of luma components or codingblocks of two chroma components and a CU of a P or B slice/tile groupmay include blocks of three color components (a luma component and twochroma components).

Although a quadtree coding tree structure with a nested multitype treehas been described, a structure in which a CU is partitioned is notlimited thereto. For example, the BT structure and the TT structure maybe interpreted as a concept included in a multiple partitioning tree(MPT) structure, and the CU may be interpreted as being partitionedthrough the QT structure and the MPT structure. In an example where theCU is partitioned through a QT structure and an MPT structure, a syntaxelement (e.g., MPT_split_type) including information on how many blocksthe leaf node of the QT structure is partitioned into and a syntaxelement (ex. MPT_split_mode) including information on which of verticaland horizontal directions the leaf node of the QT structure ispartitioned into may be signaled to determine a partitioning structure.

In another example, the CU may be partitioned in a different way thanthe QT structure, BT structure or TT structure. That is, unlike that theCU of the lower depth is partitioned into ¼ of the CU of the higherdepth according to the QT structure, the CU of the lower depth ispartitioned into ½ of the CU of the higher depth according to the BTstructure, or the CU of the lower depth is partitioned into ¼ or ½ ofthe CU of the higher depth according to the TT structure, the CU of thelower depth may be partitioned into ⅕, ⅓, ⅜, ⅗, ⅔, or ⅝ of the CU of thehigher depth in some cases, and the method of partitioning the CU is notlimited thereto.

The quadtree coding block structure with the multi-type tree may providea very flexible block partitioning structure. Because of the partitiontypes supported in a multi-type tree, different partition patterns maypotentially result in the same coding block structure in some cases. Inthe encoding apparatus and the decoding apparatus, by limiting theoccurrence of such redundant partition patterns, a data amount ofpartitioning information may be reduced.

For example, FIG. 8 shows redundant splitting patterns which may occurin binary tree splitting and ternary tree splitting. As shown in FIG. 8, continuous binary splitting 810 and 820 for one direction of two-steplevels have the same coding block structure as binary splitting for acenter partition after ternary splitting. In this case, binary treesplitting for center blocks 830 and 840 of ternary tree splitting may beprohibited. this prohibition is applicable to CUs of all pictures. Whensuch specific splitting is prohibited, signaling of corresponding syntaxelements may be modified by reflecting this prohibited case, therebyreducing the number of bits signaled for splitting. For example, asshown in the example shown in FIG. 8 , when binary tree splitting forthe center block of the CU is prohibited, a syntax elementmtt_split_cu_binary_flag specifying whether splitting is binarysplitting or ternary splitting is not signaled and the value thereof maybe derived as 0 by a decoding apparatus.

Overview of Inter Prediction

Hereinafter, inter prediction according to the present disclosure willbe described.

The prediction unit of an image encoding apparatus/image decodingapparatus according to the present disclosure may perform interprediction in units of blocks to derive a prediction sample. Interprediction may represent prediction derived in a manner that isdependent on data elements (e.g., sample values, motion information,etc.) of picture(s) other than a current picture. When inter predictionapplies to the current block, a predicted block (prediction block or aprediction sample array) for the current block may be derived based on areference block (reference sample array) specified by a motion vector ona reference picture indicated by a reference picture index. In thiscase, in order to reduce the amount of motion information transmitted inan inter prediction mode, motion information of the current block may bepredicted in units of blocks, subblocks or samples, based on correlationof motion information between a neighboring block and the current block.The motion information may include a motion vector and a referencepicture index. The motion information may further include interprediction type (L0 prediction, L1 prediction, Bi prediction, etc.)information. When applying inter prediction, the neighboring block mayinclude a spatial neighboring block present in the current picture and atemporal neighboring block present in the reference picture. A referencepicture including the reference block and a reference picture includingthe temporal neighboring block may be the same or different. Thetemporal neighboring block may be referred to as a collocated referenceblock, collocated CU (ColCU) or colBlock, and the reference pictureincluding the temporal neighboring block may be referred to as acollocated picture (colPic) or colPicture. For example, a motioninformation candidate list may be constructed based on the neighboringblocks of the current block, and flag or index information specifyingwhich candidate is selected (used) may be signaled in order to derivethe motion vector of the current block and/or the reference pictureindex.

Inter prediction may be performed based on various prediction modes. Forexample, in the case of a skip mode and a merge mode, the motioninformation of the current block may be equal to the motion informationof the selected neighboring block. In the case of the skip mode, aresidual signal may not be transmitted unlike the merge mode. In thecase of a motion information prediction (MVP) mode, the motion vector ofthe selected neighboring block may be used as a motion vector predictorand a motion vector difference may be signaled. In this case, the motionvector of the current block may be derived using a sum of the motionvector predictor and the motion vector difference. In the presentdisclosure, the MVP mode may have the same meaning as advanced motionvector prediction (AMVP).

The motion information may include L0 motion information and/or L1motion information according to the inter prediction type (L0prediction, L1 prediction, Bi prediction, etc.). The motion vector in anL0 direction may be referred to as an L0 motion vector or MVL0, and themotion vector in an L1 direction may be referred to as an L1 motionvector or MVL1. Prediction based on the L0 motion vector may be referredto as L0 prediction, prediction based on the L1 motion vector may bereferred to as L1 prediction, and prediction based both the L0 motionvector and the L1 motion vector may be referred to as Bi prediction.Here, the L0 motion vector may specify a motion vector associated with areference picture list L0 (L0) and the L1 motion vector may specify amotion vector associated with a reference picture list L1 (L1). Thereference picture list L0 may include pictures before the currentpicture in output order as reference pictures, and the reference picturelist L1 may include pictures after the current picture in output order.The previous pictures may be referred to as forward (reference) picturesand the subsequent pictures may be referred to as reverse (reference)pictures. The reference picture list L0 may further include picturesafter the current picture in output order as reference pictures. In thiscase, within the reference picture list L0, the previous pictures may befirst indexed and the subsequent pictures may then be indexed. Thereference picture list L1 may further include pictures before thecurrent picture in output order as reference pictures. In this case,within the reference picture list L1, the subsequent pictures may befirst indexed and the previous pictures may then be indexed. Here, theoutput order may correspond to picture order count (POC) order.

FIG. 9 is a flowchart illustrating an inter prediction based video/imageencoding method.

FIG. 10 is a view illustrating the configuration of an inter predictionunit 180 according to the present disclosure.

The encoding method of FIG. 9 may be performed by the image encodingapparatus of FIG. 2 . Specifically, step S610 may be performed by theinter prediction unit 180, and step S620 may be performed by theresidual processor. Specifically, step S620 may be performed by thesubtractor 115. Step S630 may be performed by the entropy encoder 190.The prediction information of step S630 may be derived by the interprediction unit 180, and the residual information of step S630 may bederived by the residual processor. The residual information isinformation on the residual samples. The residual information mayinclude information on quantized transform coefficients for the residualsamples. As described above, the residual samples may be derived astransform coefficients through the transformer 120 of the image encodingapparatus, and the transform coefficients may be derived as quantizedtransform coefficients through the quantizer 130. Information on thequantized transform coefficients may be encoded by the entropy encoder190 through a residual coding procedure.

The image encoding apparatus may perform inter prediction on a currentblock (S610). The image encoding apparatus may derive an interprediction mode and motion information of the current block and generateprediction samples of the current block. Here, inter prediction modedetermination, motion information derivation and prediction samplesgeneration procedures may be simultaneously performed or any one thereofmay be performed before the other procedures. For example, as shown inFIG. 10 , the inter prediction unit 180 of the image encoding apparatusmay include a prediction mode determination unit 181, a motioninformation derivation unit 182 and a prediction sample derivation unit183. The prediction mode determination unit 181 may determine theprediction mode of the current block, the motion information derivationunit 182 may derive the motion information of the current block, and theprediction sample derivation unit 183 may derive the prediction samplesof the current block. For example, the inter prediction unit 180 of theimage encoding apparatus may search for a block similar to the currentblock within a predetermined area (search area) of reference picturesthrough motion estimation, and derive a reference block whose adifference from the current block is equal to or less than apredetermined criterion or a minimum. Based on this, a reference pictureindex specifying a reference picture in which the reference block islocated may be derived, and a motion vector may be derived based on aposition difference between the reference block and the current block.The image encoding apparatus may determine a mode applying to thecurrent block among various prediction modes. The image encodingapparatus may compare rate-distortion (RD) costs for the variousprediction modes and determine an optimal prediction mode of the currentblock. However, the method of determining the prediction mode of thecurrent block by the image encoding apparatus is not limited to theabove example, and various methods may be used.

For example, when a skip mode or a merge mode applies to the currentblock, the image encoding apparatus may derive merge candidates fromneighboring blocks of the current block and construct a merge candidatelist using the derived merge candidates. In addition, the image encodingapparatus may derive a reference block whose a difference from thecurrent block is equal to or less than a predetermined criterion or aminimum, among reference blocks specified by merge candidates includedin the merge candidate list. In this case, a merge candidate associatedwith the derived reference block may be selected, and merge indexinformation specifying the selected merge candidate may be generated andsignaled to an image decoding apparatus. The motion information of thecurrent block may be derived using the motion information of theselected merge candidate.

As another example, when an MVP mode applies to the current block, theimage encoding apparatus may derive motion vector predictor (mvp)candidates from the neighboring blocks of the current block andconstruct an mvp candidate list using the derived mvp candidates. Inaddition, the image encoding apparatus may use the motion vector of themvp candidate selected from among the mvp candidates included in the mvpcandidate list as the mvp of the current block. In this case, forexample, the motion vector indicating the reference block derived by theabove-described motion estimation may be used as the motion vector ofthe current block, an mvp candidate with a motion vector having asmallest difference from the motion vector of the current block amongthe mvp candidates may be the selected mvp candidate. A motion vectordifference (MVD) which is a difference obtained by subtracting the mvpfrom the motion vector of the current block may be derived. In thiscase, index information specifying the selected mvp candidate andinformation on the MVD may be signaled to the image decoding apparatus.In addition, when applying the MVP mode, the value of the referencepicture index may be constructed as reference picture index informationand separately signaled to the image decoding apparatus.

The image encoding apparatus may derive residual samples based on theprediction samples (S620). The image encoding apparatus may derive theresidual samples through comparison between original samples of thecurrent block and the prediction samples. For example, the residualsample may be derived by subtracting a corresponding prediction samplefrom an original sample.

The image encoding apparatus may encode image information includingprediction information and residual information (S630). The imageencoding apparatus may output the encoded image information in the formof a bitstream. The prediction information may include prediction modeinformation (e.g., skip flag, merge flag or mode index, etc.) andinformation on motion information as information related to theprediction procedure. Among the prediction mode information, the skipflag specifies whether a skip mode applies to the current block, and themerge flag specifies whether the merge mode applies to the currentblock. Alternatively, the prediction mode information may specify one ofa plurality of prediction modes, such as a mode index. When the skipflag and the merge flag are 0, it may be determined that the MVP modeapplies to the current block. The information on the motion informationmay include candidate selection information (e.g., merge index, mvp flagor mvp index) which is information for deriving a motion vector. Amongthe candidate selection information, the merge index may be signaledwhen the merge mode applies to the current block and may be informationfor selecting one of merge candidates included in a merge candidatelist. Among the candidate selection information, the mvp flag or the mvpindex may be signaled when the MVP mode applies to the current block andmay be information for selecting one of mvp candidates in an mvpcandidate list. In addition, the information on the motion informationmay include information on the above-described MVD and/or referencepicture index information. In addition, the information on the motioninformation may include information specifying whether to apply L0prediction, L1 prediction or Bi prediction. The residual information isinformation on the residual samples. The residual information mayinclude information on quantized transform coefficients for the residualsamples.

The output bitstream may be stored in a (digital) storage medium andtransmitted to the image decoding apparatus or may be transmitted to theimage decoding apparatus via a network.

As described above, the image encoding apparatus may generate areconstructed picture (a picture including reconstructed samples and areconstructed block) based on the reference samples and the residualsamples. This is for the image encoding apparatus to derive the sameprediction result as that performed by the image decoding apparatus,thereby increasing coding efficiency. Accordingly, the image encodingapparatus may store the reconstructed picture (or the reconstructedsamples and the reconstructed block) in a memory and use the same as areference picture for inter prediction. As described above, an in-loopfiltering procedure is further applicable to the reconstructed picture.

FIG. 11 is a flowchart illustrating an inter prediction basedvideo/image decoding method.

FIG. 12 is a view illustrating the configuration of an inter predictionunit 260 according to the present disclosure.

The image decoding apparatus may perform operation corresponding tooperation performed by the image encoding apparatus. The image decodingapparatus may perform prediction on a current block based on receivedprediction information and derive prediction samples.

The decoding method of FIG. 11 may be performed by the image decodingapparatus of FIG. 3 . Steps S810 to S830 may be performed by the interprediction unit 260, and the prediction information of step S810 and theresidual information of step S840 may be obtained from a bitstream bythe entropy decoder 210. The residual processor of the image decodingapparatus may derive residual samples for a current block based on theresidual information (S840). Specifically, the dequantizer 220 of theresidual processor may perform dequantization based on dequantizedtransform coefficients derived based on the residual information toderive transform coefficients, and the inverse transformer 230 of theresidual processor may perform inverse transform on the transformcoefficients to derive the residual samples for the current block. StepS850 may be performed by the adder 235 or the reconstructor.

Specifically, the image decoding apparatus may determine the predictionmode of the current block based on the received prediction information(S810). The image decoding apparatus may determine which interprediction mode applies to the current block based on the predictionmode information in the prediction information.

For example, it may be determined whether the skip mode applies to thecurrent block based on the skip flag. In addition, it may be determinedwhether the merge mode or the MVP mode applies to the current blockbased on the merge flag. Alternatively, one of various inter predictionmode candidates may be selected based on the mode index. The interprediction mode candidates may include a skip mode, a merge mode and/oran MVP mode or may include various inter prediction modes which will bedescribed below.

The image decoding apparatus may derive the motion information of thecurrent block based on the determined inter prediction mode (S820). Forexample, when the skip mode or the merge mode applies to the currentblock, the image decoding apparatus may construct a merge candidatelist, which will be described below, and select one of merge candidatesincluded in the merge candidate list. The selection may be performedbased on the above-described candidate selection information (mergeindex). The motion information of the current block may be derived usingthe motion information of the selected merge candidate. For example, themotion information of the selected merge candidate may be used as themotion information of the current block.

As another example, when the MVP mode applies to the current block, theimage decoding apparatus may construct an mvp candidate list and use themotion vector of an mvp candidate selected from among mvp candidatesincluded in the mvp candidate list as an mvp of the current block. Theselection may be performed based on the above-described candidateselection information (mvp flag or mvp index). In this case, the MVD ofthe current block may be derived based on information on the MVD, andthe motion vector of the current block may be derived based on mvp andMVD of the current block. In addition, the reference picture index ofthe current block may be derived based on the reference picture indexinformation. A picture indicated by the reference picture index in thereference picture list of the current block may be derived as areference picture referenced for inter prediction of the current block.

The image decoding apparatus may generate prediction samples of thecurrent block based on motion information of the current block (S830).In this case, the reference picture may be derived based on thereference picture index of the current block, and the prediction samplesof the current block may be derived using the samples of the referenceblock indicated by the motion vector of the current block on thereference picture. In some cases, a prediction sample filteringprocedure may be further performed on all or some of the predictionsamples of the current block.

For example, as shown in FIG. 12 , the inter prediction unit 260 of theimage decoding apparatus may include a prediction mode determinationunit 261, a motion information derivation unit 262 and a predictionsample derivation unit 263. In the inter prediction unit 260 of theimage decoding apparatus, the prediction mode determination unit 261 maydetermine the prediction mode of the current block based on the receivedprediction mode information, the motion information derivation unit 262may derive the motion information (a motion vector and/or a referencepicture index, etc.) of the current block based on the received motioninformation, and the prediction sample derivation unit 263 may derivethe prediction samples of the current block.

The image decoding apparatus may generate residual samples of thecurrent block based the received residual information (S840). The imagedecoding apparatus may generate the reconstructed samples of the currentblock based on the prediction samples and the residual samples andgenerate a reconstructed picture based on this (S850). Thereafter, anin-loop filtering procedure is applicable to the reconstructed pictureas described above.

As described above, the inter prediction procedure may include step ofdetermining an inter prediction mode, step of deriving motioninformation according to the determined prediction mode, and step ofperforming prediction (generating prediction samples) based on thederived motion information. The inter prediction procedure may beperformed by the image encoding apparatus and the image decodingapparatus, as described above.

Hereinafter, the step of deriving the motion information according tothe prediction mode will be described in greater detail.

As described above, inter prediction may be performed using motioninformation of a current block. An image encoding apparatus may deriveoptimal motion information of a current block through a motionestimation procedure. For example, the image encoding apparatus maysearch for a similar reference block with high correlation within apredetermined search range in the reference picture using an originalblock in an original picture for the current block in fractional pixelunit, and derive motion information using the same. Similarity of theblock may be calculated based on a sum of absolute differences (SAD)between the current block and the reference block. In this case, motioninformation may be derived based on a reference block with a smallestSAD in the search area. The derived motion information may be signaledto an image decoding apparatus according to various methods based on aninter prediction mode.

When a merge mode applies to a current block, motion information of thecurrent block is not directly transmitted and motion information of thecurrent block is derived using motion information of a neighboringblock. Accordingly, motion information of a current prediction block maybe indicated by transmitting flag information specifying that the mergemode is used and candidate selection information (e.g., a merge index)specifying which neighboring block is used as a merge candidate. In thepresent disclosure, since the current block is a unit of predictionperformance, the current block may be used as the same meaning as thecurrent prediction block, and the neighboring block may be used as thesame meaning as a neighboring prediction block.

The image encoding apparatus may search for merge candidate blocks usedto derive the motion information of the current block to perform themerge mode. For example, up to five merge candidate blocks may be used,without being limited thereto. The maximum number of merge candidateblocks may be transmitted in a slice header or a tile group header,without being limited thereto. After finding the merge candidate blocks,the image encoding apparatus may generate a merge candidate list andselect a merge candidate block with smallest RD cost as a final mergecandidate block.

The present disclosure provides various embodiments for the mergecandidate blocks configuring the merge candidate list. The mergecandidate list may use, for example, five merge candidate blocks. Forexample, four spatial merge candidates and one temporal merge candidatemay be used.

FIG. 13 is a view illustrating neighboring blocks available as a spatialmerge candidate.

FIG. 14 is a view schematically illustrating a merge candidate listconstruction method according to an example of the present disclosure.

An image encoding/decoding apparatus may insert, into a merge candidatelist, spatial merge candidates derived by searching for spatialneighboring blocks of a current block (S1110). For example, as shown inFIG. 13 , the spatial neighboring blocks may include a bottom-leftcorner neighboring block A₀, a left neighboring block A₁, a top-rightcorner neighboring block B₀, a top neighboring block B₁, and a top-leftcorner neighboring block B₂ of the current block. However, this is anexample and, in addition to the above-described spatial neighboringblocks, additional neighboring blocks such as a right neighboring block,a bottom neighboring block and a bottom-right neighboring block may befurther used as the spatial neighboring blocks. The imageencoding/decoding apparatus may detect available blocks by searching forthe spatial neighboring blocks based on priority and derive motioninformation of the detected blocks as the spatial merge candidates. Forexample, the image encoding/decoding apparatus may construct a mergecandidate list by searching for the five blocks shown in FIG. 13 inorder of A₁, B₁, B₀, A₀ and B₂ and sequentially indexing availablecandidates.

The image encoding/decoding apparatus may insert, into the mergecandidate list, a temporal merge candidate derived by searching fortemporal neighboring blocks of the current block (S1120). The temporalneighboring blocks may be located on a reference picture which isdifferent from a current picture in which the current block is located.A reference picture in which the temporal neighboring block is locatedmay be referred to as a collocated picture or a col picture. Thetemporal neighboring block may be searched for in order of abottom-right corner neighboring block and a bottom-right center block ofthe co-located block for the current block on the col picture.Meanwhile, when applying motion data compression in order to reducememory load, specific motion information may be stored as representativemotion information for each predetermined storage unit for the colpicture. In this case, motion information of all blocks in thepredetermined storage unit does not need to be stored, thereby obtainingmotion data compression effect. In this case, the predetermined storageunit may be predetermined as, for example, 16×16 sample unit or 8×8sample unit or size information of the predetermined storage unit may besignaled from the image encoding apparatus to the image decodingapparatus. When applying the motion data compression, the motioninformation of the temporal neighboring block may be replaced with therepresentative motion information of the predetermined storage unit inwhich the temporal neighboring block is located. That is, in this case,from the viewpoint of implementation, the temporal merge candidate maybe derived based on the motion information of a prediction blockcovering an arithmetic left-shifted position after an arithmetic rightshift by a predetermined value based on coordinates (top-left sampleposition) of the temporal neighboring block, not a prediction blocklocated on the coordinates of the temporal neighboring block. Forexample, when the predetermined storage unit is a 2^(n)×2^(n) sampleunit and the coordinates of the temporal neighboring block are (xTnb,yTnb), the motion information of a prediction block located at amodified position ((xTnb»n)«n), (yTnb»n)«n)) may be used for thetemporal merge candidate. Specifically, for example, when thepredetermined storage unit is a 16×16 sample unit and the coordinates ofthe temporal neighboring block are (xTnb, yTnb), the motion informationof a prediction block located at a modified position ((xTnb»4)«4),(yTnb»4)«4)) may be used for the temporal merge candidate.Alternatively, for example, when the predetermined storage unit is an8×8 sample unit and the coordinates of the temporal neighboring blockare (xTnb, yTnb), the motion information of a prediction block locatedat a modified position ((xTnb»3)«3), (yTnb»3)«3)) may be used for thetemporal merge candidate.

Referring to FIG. 14 again, the image encoding/decoding apparatus maycheck whether the number of current merge candidates is less than amaximum number of merge candidates (S1130). The maximum number of mergecandidates may be predefined or signaled from the image encodingapparatus to the image decoding apparatus. For example, the imageencoding apparatus may generate and encode information on the maximumnumber of merge candidates and transmit the encoded information to theimage decoding apparatus in the form of a bitstream. When the maximumnumber of merge candidates is satisfied, a subsequent candidate additionprocess S1140 may not be performed.

When the number of current merge candidates is less than the maximumnumber of merge candidates as a checked result of step S1130, the imageencoding/decoding apparatus may derive an additional merge candidateaccording to a predetermined method and then insert the additional mergecandidate to the merge candidate list (S1140).

When the number of current merge candidates is not less than the maximumnumber of merge candidates as a checked result of step S1130, the imageencoding/decoding apparatus may end the construction of the mergecandidate list. In this case, the image encoding apparatus may select anoptimal merge candidate from among the merge candidates configuring themerge candidate list, and signal candidate selection information (e.g.,merge index) specifying the selected merge candidate to the imagedecoding apparatus. The image decoding apparatus may select the optimalmerge candidate based on the merge candidate list and the candidateselection information.

The motion information of the selected merge candidate may be used asthe motion information of the current block, and the prediction samplesof the current block may be derived based on the motion information ofthe current block, as described above. The image encoding apparatus mayderive the residual samples of the current block based on the predictionsamples and signal residual information of the residual samples to theimage decoding apparatus. The image decoding apparatus may generatereconstructed samples based on the residual samples derived based on theresidual information and the prediction samples and generate thereconstructed picture based on the same, as described above.

When applying a skip mode to the current block, the motion informationof the current block may be derived using the same method as the case ofapplying the merge mode. However, when applying the skip mode, aresidual signal for a corresponding block is omitted and thus theprediction samples may be directly used as the reconstructed samples.

When applying an MVP mode to the current block, a motion vectorpredictor (mvp) candidate list may be generated using a motion vector ofreconstructed spatial neighboring blocks (e.g., the neighboring blocksshown in FIG. 13 ) and/or a motion vector corresponding to the temporalneighboring blocks (or Col blocks). That is, the motion vector of thereconstructed spatial neighboring blocks and the motion vectorcorresponding to the temporal neighboring blocks may be used as motionvector predictor candidates of the current block. When applyingbi-prediction, an mvp candidate list for L0 motion informationderivation and an mvp candidate list for L1 motion informationderivation are individually generated and used. Prediction information(or information on prediction) of the current block may includecandidate selection information (e.g., an MVP flag or an MVP index)specifying an optimal motion vector predictor candidate selected fromamong the motion vector predictor candidates included in the mvpcandidate list. In this case, a prediction unit may select a motionvector predictor of a current block from among the motion vectorpredictor candidates included in the mvp candidate list using thecandidate selection information. The prediction unit of the imageencoding apparatus may obtain and encode a motion vector difference(MVD) between the motion vector of the current block and the motionvector predictor and output the encoded MVD in the form of a bitstream.That is, the MVD may be obtained by subtracting the motion vectorpredictor from the motion vector of the current block. The predictionunit of the image decoding apparatus may obtain a motion vectordifference included in the information on prediction and derive themotion vector of the current block through addition of the motion vectordifference and the motion vector predictor. The prediction unit of theimage encoding apparatus may obtain or derive a reference picture indexspecifying a reference picture from the information on prediction.

FIG. 15 is a view schematically illustrating a motion vector predictorcandidate list construction method according to an example of thepresent disclosure.

First, a spatial candidate block of a current block may be searched forand available candidate blocks may be inserted into an mvp candidatelist (S1210). Thereafter, it is determined whether the number of mvpcandidates included in the mvp candidate list is less than 2 (S1220)and, when the number of mvp candidates is two, construction of the mvpcandidate list may be completed.

In step S1220, when the number of available spatial candidate blocks isless than 2, a temporal candidate block of the current block may besearched for and available candidate blocks may be inserted into the mvpcandidate list (S1230). When the temporal candidate blocks are notavailable, a zero motion vector may be inserted into the mvp candidatelist, thereby completing construction of the mvp candidate list.

Meanwhile, when applying an mvp mode, a reference picture index may beexplicitly signaled. In this case, a reference picture index refidxL0for L0 prediction and a reference picture index refidxL1 for L1prediction may be distinguishably signaled. For example, when applyingthe MVP mode and applying Bi prediction, both information on refidxL0and information on refidxL1 may be signaled.

As described above, when applying the MVP mode, information on MVPderived by the image encoding apparatus may be signaled to the imagedecoding apparatus. Information on the MVD may include, for example,information specifying x and y components for an absolute value (MVDabsolute value) and a sign of the MVD. In this case, when the MVDabsolute value is greater than 0, whether the MVD absolute value isgreater than 1 and information specifying an MVD remainder may besignaled stepwise. For example, information specifying whether the MVDabsolute value is greater than 1 may be signaled only when a value offlag information specifying whether the MVD absolute value is greaterthan 0 is 1.

FIG. 16 is a view illustrating a syntax structure for transmitting MVDfrom an image encoding apparatus to an image decoding apparatusaccording to an embodiment of the present disclosure.

In FIG. 16 , abs_mvd_greater0_flag[0] specifies whether the absolutevalue of the x component of MVD is greater than 0, andabs_mvd_greater0_flag[1] specifies the absolute value of the y componentof MVD is greater than 0. Similarly, abs_mvd_greater1_flag[0] specifieswhether the absolute value of the x component of MVD is greater than 1,and abs_mvd_greater1_flag[1] specifies whether the absolute value of they component of MVD is greater than 1. As shown in FIG. 16 ,abs_mvd_greater1_flag may be transmitted only when abs_mvd_greater0_flagis 1. In FIG. 16 , abs_mvd_minus2 may specify a value obtained bysubtracting 2 from the absolute value of MVD, and mvd_sign_flag specifywhether the sign of MVD is positive or negative. Using the syntaxstructure shown in FIG. 16 , MVD may be derived as shown in Equation 1below.MVD[compIdx]=abs_mvd_greater0_flag[compIdx]*(abs_mvd_minus2[compIdx]+2)*(1−2*mvd_sign_flag[compIdx])  [Equation1]

Meanwhile, MVD (MVDL0) for L0 prediction and MVD (MVDL1) for L1prediction may be distinguishably signaled, and the information on MVDmay include information on MVDL0 and/or information on MVDL1. Forexample, when applying the MVP mode and applying BI prediction to thecurrent block, both the information on MVDL0 and the information onMVDL1 may be signaled.

Overview of Intra Block Copy (IBC) Prediction

Hereinafter, IBC prediction according to the present disclosure will bedescribed.

IBC prediction may be performed by a prediction unit of an imageencoding/decoding apparatus. IBC prediction may be simply referred to asIBC. The IBC may be used for content image/moving image coding such asscreen content coding (SCC). The IBC prediction may be basicallyperformed in the current picture, but may be performed similarly tointer prediction in that a reference block is derived within the currentpicture. That is, IBC may use at least one of inter predictiontechniques described in the present disclosure. For example, IBC may useat least one of the above-described motion information (motion vector)derivation methods. At least one of the inter prediction techniques maybe partially modified and used in consideration of the IBC prediction.The IBC may refer to a current picture and thus may be referred to ascurrent picture referencing (CPR).

For IBC, the image encoding apparatus may perform block matching (BM)and derive an optimal block vector (or motion vector) for a currentblock (e.g., a CU). The derived block vector (or motion vector) may besignaled to the image decoding apparatus through a bitstream using amethod similar to signaling of motion information (motion vector) in theabove-described inter prediction. The image decoding apparatus mayderive a reference block for the current block in the current picturethrough the signaled block vector (motion vector), and derive aprediction signal (predicted block or prediction samples) for thecurrent block through this. Here, the block vector (or motion vector)may specify displacement from the current block to a reference blocklocated in an already reconstructed area in the current picture.Accordingly, the block vector (or the motion vector) may be referred toa displacement vector. Hereinafter, in IBC, the motion vector maycorrespond to the block vector or the displacement vector. The motionvector of the current block may include a motion vector (luma motionvector) for a luma component or a motion vector (chroma motion vector)for a chroma component. For example, the luma motion vector for anIBC-coded CU may be an integer sample unit (that is, integer precision).The chroma motion vector may be clipped in integer sample units. Asdescribed above, IBC may use at least one of inter predictiontechniques, and, for example, the luma motion vector may beencoded/decoded using the above-described merge mode or MVP mode.

When applying a merge mode to the luma IBC block, a merge candidate listfor the luma IBC block may be constructed similarly to a merge candidatelist in the inter mode described with reference to FIG. 14 . However, inthe case of the luma IBC block, a temporal neighboring block may not beused as a merge candidate.

When applying the MVP mode to the luma IBC block, an mvp candidate listfor the luma IBC block may be constructed similarly to the mvp candidatelist in the inter mode described with reference to FIG. 15 . However, inthe case of the luma IBC block, a temporal candidate block may not beused as the mvp candidate.

In IBC, a reference block is derived from the already reconstructed areain the current picture. In this case, in order to reduce memoryconsumption and complexity of the image decoding apparatus, only apredefined area among already reconstructed areas in the current picturemay be referenced. The predefined area may include a current CTU inwhich the current block is included. By restricting referenceablereconstructed area to the predefined area, the IBC mode may beimplemented in hardware using a local on-chip memory.

The image encoding apparatus for performing IBC may search thepredefined area to determine a reference block with smallest RD cost andderive a motion vector (block vector) based on the positions of thereference block and the current block.

Whether to apply IBC to the current block may be signaled as IBCperformance information at a CU level. Information on a signaling method(IBC MVP mode or IBC skip/merger mode) of the motion vector of thecurrent block may be signaled. IBC performance information may be usedto determine the prediction mode of the current block. Accordingly, theIBC performance information may be included in information on theprediction mode of the current block.

In the case of the IBC skip/merge mode, a merge candidate index may besignaled to specify a block vector to be used for prediction of thecurrent luma block among block vectors included in the merge candidatelist. In this case, the merge candidate list may include IBC-encodedneighboring blocks. The merge candidate list may be configured toinclude spatial merge candidates and not to include temporal mergecandidates. In addition, the merge candidate list may further includehistory-based motion vector predictor (HMVP) candidates and/or pairwisecandidates.

In the case of the IBC MVP mode, a block vector difference value may beencoded using the same method as a motion vector difference value of theabove-described inter mode. The block vector prediction method mayconstruct and use an mvp candidate list including two candidates aspredictors similarly to the MVP mode of the inter mode. One of the twocandidates may be derived from a left neighboring block and the othercandidate may be derived from a top neighboring block. In this case,only when the left or top neighboring block is IBC-encoded, candidatesmay be derived from the corresponding neighboring block. If the left ortop neighboring block is not available, for example, is not IBC-encoded,a default block vector may be included in the mvp candidate list as apredictor. In addition, information (e.g., flag) specifying one of twoblock vector predictors is signaled and used as candidate selectioninformation similarly to the MVP mode of the inter mode. The mvpcandidate list may include an HMVP candidate and/or a zero motion vectoras the default block vector.

The HMVP candidate may be referred to as a history-based MVP candidate,and an MVP candidate used before encoding/decoding of the current block,a merge candidate or a block vector candidate may be stored in an HMVPlist as HMVP candidates. Thereafter, when the merge candidate list ofthe current block or the mvp candidate list does not include a maximumnumber of candidates, candidates stored in the HMVP list may be added tothe merge candidate list or mvp candidate list of the current block asHMVP candidates.

The pairwise candidate means a candidate derived by selecting twocandidates according to a predetermined order from among candidatesalready included in the merge candidate list of the current block andaveraging the selected two candidates.

FIG. 17 is a flowchart illustrating an IBC based video/image encodingmethod.

FIG. 18 is a view illustrating the configuration of a prediction unitfor performing an IBC based video/image encoding method according to thepresent disclosure.

The encoding method of FIG. 17 may be performed by the image encodingapparatus of FIG. 2 . Specifically, step S1410 may be performed by theprediction unit and step S1420 may be performed by the residualprocessor. Specifically, step S1420 may be performed by the subtractor115. Step S1430 may be performed by the entropy encoder 190. Theprediction information of step S1430 may be derived by the predictionunit and the residual information of step S1430 may be derived by theresidual processor. The residual information is information on theresidual samples. The residual information may include information onquantized transform coefficients for the residual samples. As describedabove, the residual samples may be derived by the transform coefficientsthrough the transformer 120 of the image encoding apparatus, and thetransform coefficients may be derived by transform coefficientsquantized through the quantizer 130. Information on the quantizedtransform coefficients may be encoded by the entropy encoder 190 througha residual coding procedure.

The image encoding apparatus may perform IBC prediction (IBC basedprediction) for the current block (S1410). The image encoding apparatusmay derive a prediction mode and motion vector (block vector) of thecurrent block and generate prediction samples of the current block. Theprediction mode may include at least one of the above-described interprediction modes. Here, prediction mode determination, motion vectorderivation and prediction samples generation procedures may besimultaneously performed or any one procedure may be performed beforethe other procedures. For example, as shown in FIG. 18 , the predictionunit of the image encoding apparatus for performing an IBC-basedvideo/image encoding method may include a prediction mode determinationunit, a motion vector derivation unit and a prediction sample derivationunit. The prediction mode determination unit may determine theprediction mode of the current block, the motion vector derivation unitmay derives the motion vector of the current block, and the predictionsample derivation unit may derive the prediction samples of the currentblock. For example, the prediction unit of the image encoding apparatusmay search for a block similar to the current block in a reconstructedarea (or a certain area (search area) of the reconstructed area) of acurrent picture and derive a reference block whose a difference from thecurrent block is equal to or less than a certain criterion or a minimum.The image encoding apparatus may derive a motion vector based on adisplacement difference between the reference block and the currentblock. The image encoding apparatus may determine a mode applying to thecurrent block among various prediction modes. The image encodingapparatus may compare RD costs for the various prediction modes anddetermine an optimal prediction mode for the current block. However, amethod of determining the prediction mode for the current block by theimage encoding apparatus is not limited to the above example and variousmethods may be used.

For example, when applying a skip mode or a merge mode to the currentblock, the image encoding apparatus may derive merge candidates fromneighboring blocks of the current block and construct a merge candidatelist using the derived merge candidates. In addition, the image encodingapparatus may derive a reference block whose a difference from thecurrent block is equal to or less than a certain criterion or a minimumamong reference blocks indicated by the merge candidates included in themerge candidate list. In this case, a merge candidate associated withthe derived reference block may be selected and merge index informationspecifying the selected merge candidate may be generated and signaled tothe image decoding apparatus. Using the motion vector of the selectedmerge candidate, the motion vector of the current block may be derived.

As another example, when applying an MVP mode to the current block, theimage encoding apparatus may derive motion vector predictor (mvp)candidates from the neighboring blocks of the current block andconstruct an mvp candidate list using the derived mvp candidates. Inaddition, the image encoding apparatus may use the motion vector of themvp candidate selected from among the mvp candidates included in the mvpcandidate list as the mvp of the current block. In this case, forexample, a motion vector indicating a reference block derived by theabove-described motion estimation may be used as the motion vector ofthe current block, and an mvp candidate having a smallest differencefrom the motion vector of the current block among the mvp candidates maybecome the selected mvp candidate. A motion vector difference (MVD)which is obtained by subtracting the mvp from the motion vector of thecurrent block may be derived. In this case, index information specifyingthe selected mvp candidate and information on the MVD may be signaled tothe image decoding apparatus.

The image encoding apparatus may derive residual samples based on theprediction samples (S1420). The image encoding apparatus may derive theresidual samples through comparison between the original samples of thecurrent block and the prediction samples. For example, the residualsample may be derived by subtracting the corresponding prediction samplefrom the original sample.

The image encoding apparatus may encode image information includingprediction information and residual information (S1430). The imageencoding apparatus may output the encoded image information in the formof a bitstream. The prediction information may include prediction modeinformation (e.g., skip flag, merge flag or mode index) and informationon a motion vector as information related to the prediction procedure.Among the prediction mode information, the skip flag specifies whetherto apply the skip mode to the current block and the merge flag specifieswhether to apply the merge mode to the current block. Alternatively, theprediction mode information may specify one of a plurality of predictionmodes, such as a mode index. When the skip flag and the merge flag are0, it may be determined that the MVP mode applies to the current block.The information on the motion vector may include candidate selectioninformation (e.g., merge index, mvp flag or mvp index) which isinformation for deriving the motion vector. Among the candidateselection information, the merge index may be signaled when applying themerge mode to the current block and may be information for selecting oneof the merge candidates included in the merge candidate list. Among thecandidate selection information, the mvp flag or mvp index may besignaled when applying the MVP mode to the current block and may beinformation for selecting one of the mvp candidates included in the mvpcandidate list. In addition, the information on the motion vector mayinclude information on the above-described MVD. In addition, theinformation on the motion vector may include information specifyingwhether to apply L0 prediction, L1 prediction or bi prediction. Theresidual information is information on the residual samples. Theresidual information may include information on the quantized transformcoefficients for the residual samples.

The output bitstream may be stored in a (digital) storage medium andtransmitted to the image decoding apparatus or may be transmitted to theimage decoding apparatus through a network.

Meanwhile, as described above, the image encoding apparatus may generatea reconstructed picture (picture including reconstructed samples and areconstructed block) based on the reference samples and the residualsamples. This is for the image encoding apparatus to derive the sameprediction result as that performed by the image decoding apparatus,thereby increasing coding efficiency. Accordingly, the image encodingapparatus may store the reconstructed picture (or reconstructed samplesand reconstructed block) in a memory and use the same as a referencepicture for inter prediction. An in-loop filtering procedure is furtherapplicable to the reconstructed picture, as described above.

FIG. 19 is a flowchart illustrating an IBC based video/image decodingmethod.

FIG. 20 is a view illustrating a configuration of a prediction unit forperforming an IBC based video/image decoding method according to thepresent disclosure.

The image decoding apparatus may perform operation corresponding tooperation performed by the image encoding apparatus. The image decodingapparatus may perform IBC prediction for a current block based onreceived prediction information to derive prediction samples.

The decoding method of FIG. 19 may be performed by the image decodingapparatus of FIG. 3 . Steps S1610 to S1630 may be performed by theprediction unit and the prediction information of step S1610 and theresidual information of step S1640 may be obtained from a bitstream bythe entropy decoder 210. The residual processor of the image decodingapparatus may derive residual samples for the current block based on theresidual information (S1640). Specifically, the dequantizer 220 of theresidual processor may perform dequantization based on the quantizedtransform coefficient derived based on the residual information toderive transform coefficients, and the inverse transformer 230 of theresidual processor may perform inverse transform on the transformcoefficients to derive residual samples for the current block. StepS1650 may be performed by the adder 235 or the reconstructor.

Specifically, the image decoding apparatus may determine the predictionmode of the current block based on the received prediction information(S1610). The image decoding apparatus may determine which predictionmode applies to the current block based on the prediction modeinformation in the prediction information.

For example, it may be determined whether to apply the skip mode to thecurrent block based on the skip flag. In addition, it may be determinedwhether to apply the merge node or MVP mode to the current block basedon the merge flag. Alternatively, one of various prediction modecandidates may be selected based on the mode index. The prediction modecandidates may include a skip mode, a merge mode and/or an MVP mode ormay include the above-described various inter prediction modes.

The image encoding apparatus may derive the motion vector of the currentblock based on the determined prediction mode (S1620). For example, whenapplying the skip mode or the merge mode to the current block, the imagedecoding apparatus may construct the above-described merge candidatelist and select one of the merge modes included in the merge candidatelist. The selection may be performed based on the above-describedcandidate selection information (merge index). The motion vector of thecurrent block may be derived using the motion vector of the selectedmerge candidate. For example, the motion vector of the selected mergecandidate may be used as the motion vector of the current block.

As another example, when applying the MVP mode to the current block, theimage decoding apparatus may construct an mvp candidate list and use themotion vector of the mvp candidate selected from among the mvpcandidates included in the mvp candidate list as the mvp of the currentblock. The selection may be performed based on the above-describedcandidate selection information (mvp flag or mvp index). In this case,the MVD of the current block may be derived based on information on theMVD, and the motion vector of the current block may be derived based onthe mvp and MVD of the current block.

The image decoding apparatus may generate prediction samples of thecurrent block based on the motion vector of the current block (S1630).The prediction samples of the current block may be derived using thesamples of the reference block indicated by the motion vector of thecurrent block on the current picture. In some cases, a prediction samplefiltering procedure for all or some of the prediction samples of thecurrent block may be further performed.

For example, as shown in FIG. 20 , the prediction unit of the imagedecoding apparatus for performing an IBC based video/image decodingmethod may include a prediction mode determination unit, a motion vectorderivation unit and a prediction sample derivation unit. The predictionunit of the image decoding apparatus may determine the prediction modefor the current block based on the received prediction mode informationin the prediction mode determination unit, derive the motion vector ofthe current block based on the received information on the motion vectorin the motion vector derivation unit, and derive the prediction samplesof the current block in the prediction sample derivation unit.

The image decoding apparatus may generate residual samples of thecurrent block based on the received residual information (S1640). Theimage decoding apparatus may generate reconstructed samples for thecurrent block based on the prediction samples and the residual samples,and generate a reconstructed picture based on this (S1650). Thereafter,an in-loop filtering procedure is further applicable to thereconstructed picture, as described above.

As described above, one unit (e.g., a coding unit (CU)) may include aluma block (luma coding block (CB)) and a chroma block (chroma CB). Inthis case, the luma block and the chroma block corresponding thereto mayhave the same motion information (e.g., motion vector) or differentmotion information. For example, the motion information of the chromablock may be derived based on the motion information of the luma block,such that the luma block and the chroma block corresponding thereto havethe same motion information.

Overview of Chroma Format

Hereinafter, a chroma format will be described. An image may be encodedinto encoded data including a luma component (e.g., Y) array and twochroma component (e.g., Cb and Cr) arrays. For example, one pixel of theencoded image may include a luma sample and a chroma sample. A chromaformat may be used to represent a configuration format of the lumasample and the chroma sample, and the chroma format may be referred toas a color format.

In an embodiment, an image may be encoded into various chroma formatssuch as monochrome, 4:2:0, 4:2:2 or 4:4:4. In monochrome sampling, theremay be one sample array and the sample array may be a luma array. In4:2:0 sampling, there may be one luma sample array and two chroma samplearrays, each of the two chroma arrays may have a height equal to halfthat of the luma array and a width equal to half that of the luma array.In 4:2:2 sampling, there may be one luma sample array and two chromasample arrays, each of the two chroma arrays may have a height equal tothat of the luma array and a width equal to half that of the luma array.In 4:4:4 sampling, there may be one luma sample array and two chromasample arrays, and each of the two chroma arrays may have a height andwidth equal to those of the luma array.

For example, in 4:2:0 sampling, a chroma sample may be located below aluma sample corresponding thereto. In 4:2:2 sampling, a chroma samplemay be located to overlap a luma sample corresponding thereto. In 4:4:4sampling, both a luma sample and a chroma sample may be located at anoverlapping position.

A chroma format used in an encoding apparatus and a decoding apparatusmay be predetermined. Alternatively, a chroma format may be signaledfrom an encoding apparatus to a decoding apparatus to be adaptively usedin the encoding apparatus and the decoding apparatus. In an embodiment,the chroma format may be signaled based on at least one ofchroma_format_idc or separate_colour_plane_flag. At least one ofchroma_format_idc or separate_colour_plane_flag may be signaled throughhigher level syntax such as DPS, VPS, SPS or PPS. For example,chroma_format_idc and separate_colour_plane_flag may be included in SPSsyntax shown in FIG. 21 .

Meanwhile, FIG. 22 shows an embodiment of chroma format classificationusing signaling of chroma_format_idc and separate_colour_plane_flag.chroma_format_idc may be information specifying a chroma format applyingto an encoded image. separate_colour_plane_flag may specify whether acolor array is separately processed in a specific chroma format. Forexample, a first value (e.g., 0) of chroma_format_idc may specifymonochrome sampling. A second value (e.g., 1) of chroma_format_idc mayspecify 4:2:0 sampling. A third value (e.g., 2) of chroma_format_idc mayspecify 4:2:2 sampling. A fourth value (e.g., 3) of chroma_format_idcmay specify 4:4:4 sampling.

In 4:4:4, the following may apply based on the value ofseparate_colour_plane_flag. If the value of separate_colour_plane_flagis a first value (e.g., 0), each of two chroma arrays may have the sameheight and width as a luma array. In this case, a value ofChromaArrayType specifying a type of a chroma sample array may be setequal to chroma_format_idc. If the value of separate_colour_plane_flagis a second value (e.g., 1), luma, Cb and Cr sample arrays may beseparately processed and processed along with monochrome-sampledpictures. In this case, ChromaArrayType may be set to 0.

Intra Prediction on Chroma Block

When intra prediction is performed on a current block, prediction on aluma component block (luma block) of the current block and prediction ona chroma component block (chroma block) may be performed. In this case,the intra prediction mode for the chroma block may be set separatelyfrom the intra prediction mode for the luma block.

For example, the intra prediction mode for the chroma block may bespecified based on intra chroma prediction mode information, and theintra chroma prediction mode information may be signaled in the form ofan intra_chroma_pred_mode syntax element. For example, the intra chromaprediction mode information may represent one of a planar mode, a DCmode, a vertical mode, a horizontal mode, a derived mode (DM), and across-component linear model (CCLM) mode. Here, the planar mode mayspecify intra prediction mode #0, the DC mode may specify intraprediction mode #1, the vertical mode may specify intra prediction mode#26, and the horizontal mode may specify intra prediction mode #10. DMmay also be referred to as a direct mode. The CCLM may also be referredto as a linear model (LM).

Meanwhile, the DM and the CCLM are dependent intra prediction modes forpredicting the chroma block using information on the luma block. The DMmay represent a mode in which the same intra prediction mode as theintra prediction mode for the luma component applies as the intraprediction mode for the chroma component. In addition, the CCLM mayrepresent an intra prediction mode using, as the prediction samples ofthe chroma block, samples derived by subsampling reconstructed samplesof the luma block and then applying α and β which are CCLM parameters tosubsampled samples in a process of generating the prediction block forthe chroma block.pred_(c)(i,j)=α·rec _(L)′(i,j)+β  [Equation 2]

where, pred_(c)(i,j) may denote the prediction sample of (i, j)coordinates of the current chroma block in the current CU. rec_(L)′(i,j)may denote the reconstructed sample of (i, j) coordinates of the currentluma block in the CU. For example, rec_(L)′(i,j) may denote thedown-sampled reconstructed sample of the current luma block. Linearmodel coefficients α and β may be signaled or derived from neighboringsamples.

Virtual Pipeline Data Unit

Virtual pipeline data units (VPDUs) may be defined for pipelineprocessing within a picture. The VPDUs may be defined as non-overlappingunits within one picture. In a hardware decoding apparatus, successiveVPDUs may be simultaneously processed by multiple pipeline stages. Inmost pipeline stages, a VPDU size may be roughly proportional to abuffer size. Accordingly, keeping the VPDU size small is important whenconsidering the buffer size from a point of view of hardware. In mosthardware decoding apparatuses, the VPDU size may be set equal to amaximum transform block (TB) size. For example, the VPDU size may be64×64 (64×64 luma samples) size. Alternatively, in VVC, the VPDU sizemay be changed (increased or decreased) in consideration of theabove-described ternary tree (TT) and/or binary tree (BT) partition.

Meanwhile, to keep the VPDU size at the sample of 64×64 luma samples,splitting of a CU shown in FIG. 23 may be restricted. More specifically,at least one of the following restrictions may be applied.

Restriction 1: Ternary tree (TT) splitting is not allowed for a CUhaving a width or height of 128 or a width and height of 128.

Restriction 2: Horizontal binary tree (BT) splitting is not allowed fora CU having 128×N (where, N is an integer equal to or less than 64 andgreater than 0) (e.g., horizontal binary tree splitting is not allowedfor a CU having a width of 128 and a height less than 128).

Restriction 3: Vertical binary tree (BT) splitting is not allowed for aCU having N×128 (where, N is an integer equal to or less than 64 andgreater than 0) (e.g., vertical binary tree splitting is not allowed fora CU having a height of 128 and a width less than 128).

Maximum Size Limitation Problem of Chroma Block for Pipeline Processing

As described above with respect to a partitioning structure and atransform process, a CU may be split to generate a plurality of TUs.When a size of the CU is greater than a maximum TU size, the CU may besplit into a plurality of TUs. Therefore, transform and/or inversetransform may be performed on each TU. In general, a maximum TU size fora luma block may be set to a maximum available transform size which maybe performed by an encoding apparatus and/or a decoding apparatus.Examples of splitting a CU and a TU according to an embodiment are shownin FIGS. 24 to 26 .

FIG. 24 shows an example of a TU generated by splitting a luma CU and achroma CU according to an embodiment. In an embodiment, a maximum sizeof the luma CU may be 64×64, a maximum available transform size may be32×32, and a non-square TU may not be allowed. Therefore, a maximum sizeof a luma component transform block may be 32×32. In this embodiment,the maximum TU size may be set as shown in the following equation.maxTbSize=(cIdx==0)?MaxTbSizeY:MaxTbSizeY/max(SubWidthC,SubHeightC)  [Equation 3]

In the above equation, maxTbSize may be a maximum size of a transformblock (TB), and cIdx may be a color component of a corresponding block.cIdx 0 may denote a luma component, 1 may denote a Cb chroma component,and 2 may denote a Cr chroma component. MaxTbSizeY may denote a maximumsize of a luma component transform block, SubWidthC may denote a ratioof the width of the luma block to the width of the chroma block,SubHeightC may denote a ratio of the height of the luma block to theheight of the chroma block, and max(A, B) may denote a function forreturning the larger value of A and B as a result value.

According to the above equation, in the above-described embodiment, inthe case of the luma block, a maximum size of a transform block may beset as a maximum size of a luma component transform block. Here, themaximum size of the luma component transform block is a value set duringencoding and may be signaled from the encoding apparatus to the decodingapparatus through a bitstream.

In addition, in the above embodiment, a maximum size of a transformblock of the chroma block may be set to a value obtained by dividing themaximum size of the luma component transform block by the larger valueof SubWidthC and SubHeightC. Here, SubWidthC and SubHeightC may bedetermined based on chroma_format_idc and separate_colour_plane_flagsignaled from the encoding apparatus to the decoding apparatus through abitstream as shown in FIG. 23 .

According to the above Equation, in the above embodiment, the maximumsize of the transform block may be determined to be any one of a minimumwidth and a minimum height of the transform block. Therefore, TUsplitting of the luma block and the chroma block in the above embodimentmay be performed as shown in FIG. 24 . For example, as shown in FIG. 24, in the case of a chroma block having 4:2:2 format, as the maximum sizeof the transform block is determined to be 16, a chroma CU may be splitinto a plurality of transform blocks in the form different from the formof splitting of a luma CU into transform blocks.

FIG. 25 shows an example of a TU generated by splitting a luma CU and achroma CU according to another embodiment. In an embodiment, a maximumsize of the luma CU may be 128×128, a maximum available transform sizemay be 64×64, and a non-square TU may not be allowed. Therefore, themaximum size of a luma component transform block may be 64×64. In thisembodiment, the maximum size of the transform block may be set as shownin the following equation. In the following equation, min(A, B) may be afunction for returning the smaller value of A and B.maxTbSize=(cIdx==0)?MaxTbSizeY:MaxTbSizeY/min(SubWidthC,SubHeightC)  [Equation 4]

Meanwhile, according to the above equation, as the larger value of thewidth and height of the corresponding block applies as the maximum sizeof the transform block, the luma CU and the chroma CU may be split intoa plurality of TUs as in the example of FIG. 25 .

In the examples of FIGS. 24 and 25 , in splitting a chroma CU having a4:2:2 format into TUs, it is split in the form different from the formof splitting the corresponding luma CU into TUs. However, whenencoding/decoding of the chroma block is performed with reference to theluma block as in a DM mode or CCLM mode for prediction of the chromablock described above, performing encoding (or decoding) of thecorresponding chroma block immediately after encoding (or decoding) ofthe luma block corresponding to the chroma block is efficient in orderto reduce delay in pipeline processing and to save a memory.

However, in the example of FIG. 24 , encoding of two chroma blocks 2421and 2423 shall be performed after encoding of one luma transform block2411, which requires separate processing in a relationship withdifferent color formats (4:4:4 or 4:2:0). In addition, in the example ofFIG. 25 , encoding of one chroma transform block 2521 shall be performedafter encoding of two luma transform blocks 2511 and 2512. In this way,in the above-described TU splitting method, when a 4:2:2 format is used,since a luma block and a chroma block corresponding thereto do notmatch, a separate process may be added to perform pipeline processing orpipeline processing may not be performed.

Maximum Size Limitation of Chroma Transform Block for PipelineProcessing

Hereinafter, a method of setting a size of a maximum transform block fora chroma CU to satisfy a condition for performing the above-describedVPDU will be described.

FIG. 26 shows an example of a TU generated by splitting a luma CU and achroma CU according to another embodiment. In an embodiment, a maximumsize of a luma CU may be 128×128, a maximum available transform size maybe 64×64, and splitting into non-square TU may be allowed. Therefore, amaximum size of a luma component transform block may be 64×64.

As shown in FIG. 26 , for splitting into non-square TUs, the maximumsize of the transform block may be defined for the width and the height.For example, the maximum width (maxTbWidth) of the transform block andthe maximum height (maxTbHeight) of the transform block may be definedas shown in the following equations, thereby defining the maximum sizeof the transform block.maxTbWidth=(cIdx==0)?MaxTbSizeY:MaxTbSizeY/SubWidthC  [Equation 5]maxTbHeight=(cIdx==0)?MaxTbSizeY:MaxTbSizeY/SubHeightC  [Equation 6]

As in the above embodiment, by defining the maximum size of thetransform block as the width and the height, as shown in the example ofFIG. 26 , even in the case of a chroma CU having a 4:2:2 format, as inthe form of splitting of the corresponding luma CU into TUs, the chromaCU may be split into TUs. Therefore, by splitting the chroma CU into TUsto correspond to the TUs of the luma CU, immediately after encoding (ordecoding) of the luma block, encoding (or decoding) of the chroma blockcorresponding thereto may be performed, thereby reducing delay inpipeline processing.

Maximum Size Limitation of Chroma Transform Block in Inter PredictionMode and IBC Prediction Mode

Hereinafter, performing of an inter prediction mode and an IBCprediction mode, to which maximum size limitation of a chroma transformblock for the above-described chroma pipeline processing applies, willbe described. An encoding apparatus and a decoding apparatus may performinter prediction and IBC prediction by limiting the maximum size of thechroma transform block according to the following description andoperations thereof may correspond to each other. In addition, thefollowing description of inter prediction may apply to the IBCprediction mode with change. Therefore, hereinafter, inter predictionoperation of a decoding apparatus according to an embodiment will bedescribed.

The decoding apparatus according to an embodiment may generate a lumaprediction block predSamplesL having a size of (cbWidth)×(cbHeight) andchroma prediction blocks predSamplesCb and predSamplesCr having a sizeof (cbWidth/SubWidthC)×(cbHeight/SubHeightC), by performing interprediction. Here, cbWidth may be the width of a current CU and cbHeightmay be the height of the current CU.

In addition, the decoding apparatus may generate a luma residual blockresSamplesL having a size of (cbWidth)×(cbHeight) and chroma residualblocks resSamplesCr and resSamplesCb having a size of(cbWidth/SubWidthC)×(cbHeight/SubHeightC). Finally, the decodingapparatus may generate a reconstructed block using the prediction blocksand the residual blocks.

Hereinafter, a method of limiting a maximum size of a chroma transformblock to generate a residual block of a CU encoded in an interprediction mode by a decoding apparatus according to an embodiment willbe described. The decoding apparatus may generate a reconstructed blockusing the residual block generated in this step.

The decoding apparatus according to an embodiment may directly obtainthe following information from a bitstream or derive the followinginformation from other information obtained from the bitstream togenerate a residual block having a size of (nTbW)×(nTbH) of a CU encodedin an inter prediction mode. Here, nTbW and nTbH may be set to the widthcbWidth of the current CU and the height cbHeight of the current CU.

-   -   Sample position (xTb0, yTb0) specifying the position of the        top-left sample of a current transform block relative to the        position of the top-left sample of a current picture    -   Parameter nTbW specifying the width of the current transform        block    -   Parameter nTbH specifying the height of the current transform        block    -   Parameter cIdx specifying a color component of the current CU

The decoding apparatus may derive a maximum width maxTbWidth of atransform block and a maximum height maxTbHeight of the transform blockfrom the received information as follows.maxTbWidth=(cIdx==0)?MaxTbSizeY:MaxTbSizeY/SubWidthC   [Equation 7]maxTbHeight=(cIdx==0)?MaxTbSizeY:MaxTbSizeY/SubHeightC  [Equation 8]

Furthermore, the decoding apparatus may derive the top-left sampleposition (xTbY, yTbY) of the current transform block based on whetherthe current CU is a luma component or a chroma component as follows.(xTbY,yTbY)=(cIdx==0)?(xTb0,yTb0):(xTb0*SubWidthC,yTb0*SubHeightC)  [Equation 9]

As in the above equation, when a current transform block is a chromablock, in order to reflect the size of a chroma block determinedaccording to the chroma format of the current transform block, themaximum width and height of the transform block and the top-left sampleposition of the current transform block may be determined based on thechroma format.

Hereinafter, the decoding apparatus may generate a residual block byperforming the following procedure. This will be described withreference to FIG. 27 . First, the decoding apparatus may determinewhether the current transform block is split (S2710). For example, thedecoding apparatus may determine whether the current transform block issplit based on whether the width and height of the current transformblock are greater than the width and height of the maximum transformblock. For example, when nTbW is greater than maxTbWidth or nTbH isgreater than maxTbHeight, the decoding apparatus may determine thatlow-layer transform blocks are generated by splitting the currenttransform block.

When the current transform block is split into lower-layer transformblocks, as described above, the decoding apparatus may derive the widthnewTbW of the lower-layer transform block and the height newTbH of thelower-layer transform block as shown in the following equation (S2720).newTbW=(nTbW>maxTbWidth)?(nTbW/2):nTbW  [Equation 10]newTbH=(nTbH>maxTbHeight)?(nTbH/2):nTbH  [Equation 11]

Next, the decoding apparatus may generate a residual block using thelower-layer transform blocks split from the current transform block(S2730). In an embodiment, as shown in FIG. 26 , the current transformblock may be a transform block having the width and height of a chromaCU having a 4:2:2 format, and the lower-layer transform blocks may be afirst lower-layer transform block 2621 to a fourth lower-layer block2624 obtained by splitting the current transform block in four in thenon-square form.

First, the decoding apparatus may generate a residual block for thefirst lower-layer transform block. Referring to FIG. 26 , the firstlower-layer transform block 2621 may be specified by the sample position(xTb0, yTb0), the width newTbW of the lower-layer transform block andthe height newTbH of the lower-layer transform block. The decodingapparatus may generate the residual block of the first lower-layertransform block 2621 using a color component cIdx of a current CU. Basedon this, the decoding apparatus may generate a modified reconstructedpicture. Thereafter, in-loop filtering may be performed for the modifiedreconstructed picture.

Next, when nTbW is greater than maxTbWidth, the decoding apparatus maygenerate a residual block for the second lower-layer transform block.The second lower-layer transform block 2622 may be specified by a sampleposition (xTb0+newTbW, yTb0), the width newTbW of the lower-layertransform block and the height newTbH of the lower-layer transformblock. The decoding apparatus may generate the residual block of thesecond lower-layer transform block 2622 using the color component cIdxof the current CU. Based on this, the decoding apparatus may generate amodified reconstructed picture. Thereafter, in-loop filtering may beperformed for the modified reconstructed picture.

Next, when nTbH is greater than maxTbHeight, the decoding apparatus maygenerate a residual block for the third lower-layer transform block. Thethird lower-layer transform block 2623 may be specified by a sampleposition (xTb0, yTb0+newTbH), the width newTbW of the lower-layertransform block and the height newTbH of the lower-layer transformblock. Similarly to the above description, the decoding apparatus maygenerate the residual block using the color component cIdx of thecurrent CU.

Next, when nTbW is greater than maxTbWidth and nTbH is greater thanmaxTbHeight, the decoding apparatus may generate a residual block forthe fourth lower-layer transform block. The fourth lower-layer transformblock 2624 may be specified by a sample position (xTb0+newTbW,yTb0+newTbH), the width newTbW of the lower-layer transform block andthe height newTbH of the lower-layer transform block. Similarly to theabove description, the decoding apparatus may generate the residualblock using the color component cIdx of the current CU.

Meanwhile, when splitting of the current transform block is notperformed, the decoding apparatus may perform inter prediction asfollows. For example, when nTbW is less than maxTbWidth and nTbH is lessthan maxTbHeight, the current transform block may not be split. In thiscase, the decoding apparatus may generate a residual block for an interprediction mode, by performing a scaling and transform process using asample position (xTbY, xTbY), a color component cIdx of the current CU,a transform block width nTbW, and a transform block height nTbH asinput. Based on this, the decoding apparatus may generate a modifiedreconstructed picture. Thereafter, in-loop filtering may be performedfor the modified reconstructed picture.

Maximum Size Limitation of Chroma Transform Block in Intra PredictionMode

Hereinafter, performing of an intra prediction mode, to which maximumsize limitation of a chroma transform block for the above-describedchroma pipeline processing applies, will be described. An encodingapparatus and a decoding apparatus may perform intra prediction bylimiting the maximum size of the chroma transform block according to thefollowing description and operations thereof may correspond to eachother. Therefore, hereinafter, operation of the decoding apparatus willbe described.

The decoding apparatus according to an embodiment may generate areconstructed picture, by performing intra prediction. In-loop filteringmay be performed on the reconstructed picture. The decoding apparatusaccording to an embodiment may directly obtain the following informationfrom a bitstream or derive the following information from otherinformation obtained from the bitstream to perform intra prediction.

-   -   Sample position (xTb0, yTb0) specifying the position of the        top-left sample of a current transform block relative to the        position of the top-left sample of a current picture    -   Parameter nTbW specifying the width of the current transform        block    -   Parameter nTbH specifying the height of the current transform        block    -   Parameter predModeIntra specifying the intra prediction mode of        the current CU    -   Parameter cIdx specifying a color component of the current CU

The decoding apparatus may derive a maximum width maxTbWidth of atransform block and a maximum height maxTbHeight of the transform blockfrom the received information as follows.maxTbWidth=(cIdx==0)?MaxTbSizeY:MaxTbSizeY/SubWidthC  [Equation 12]maxTbHeight=(cIdx==0)?MaxTbSizeY:MaxTbSizeY/SubHeightC  [Equation 13]

Furthermore, the decoding apparatus may derive the top-left sampleposition (xTbY, yTbY) of the current transform block based on whetherthe current CU is a luma component or a chroma component as follows.(xTbY,yTbY)=(cIdx==0)?(xTb0,yTb0):(xTb0*SubWidthC,yTb0*SubHeightC)  [Equation14]

Hereinafter, the decoding apparatus may perform intra prediction byperforming the following procedure. This will be described withreference to FIG. 28 . First, the decoding apparatus may determinewhether the current transform block is split (S2810). For example, thedecoding apparatus may determine whether the current transform block issplit based on whether the width and height of the current transformblock are greater than the width and height of the maximum transformblock. In addition, the decoding apparatus may determine whethersplitting is performed by further considering whether intrasub-partition (ISP) applies to the current CU. For example, when nTbW isgreater than maxTbWidth or nTbH is greater than maxTbHeight, thedecoding apparatus may determine that intra prediction is performed bysplitting the current transform block. In addition, even in this case,the decoding apparatus may determine that intra prediction is performedby splitting the current transform block, only when ISP does not applyto the current CU (e.g., the value of IntraSubpartitonSplitType isNO_ISP_SPLIT, that is, ISP does not apply to the current CU).

When the current transform block is split into lower-layer transformblocks, the decoding apparatus may derive the width newTbW of thelower-layer transform block and the height newTbH of the lower-layertransform block as shown in the following equation (S2820).newTbW=(nTbW>maxTbWidth)?(nTbW/2):nTbW  [Equation 15]newTbH=(nTbH>maxTbHeight)?(nTbH/2):nTbH  [Equation 16]

It will be described with reference to FIG. 26 . In an embodiment, thewidth nTbW of the current transform block may be the width of the chromaCU, and the height nTbH of the current transform block may be the heightof the chroma CU. In this embodiment, the width newTbW of thelower-layer transform block and the height newTbH of the lower-layertransform block may be determined to be the width and height of thetransform block 2621 split from the chroma CU. That is, in thisembodiment, the current transform block may be a transform block havingthe width and height of the chroma CU having a 4:2:2 format, and thelower-layer transform blocks may be a first lower-layer transform block2621 to a fourth lower-layer block 2624 obtained by splitting thecurrent transform block in four in the non-square form.

Next, the decoding apparatus may perform intra prediction using thelower-layer transform blocks split from the current transform block(S2830). First, the decoding apparatus may perform intra prediction onthe first lower-layer transform block. Referring to FIG. 26 , the firstlower-layer transform block 2621 may be specified by the sample position(xTb0, yTb0), the width newTbW of the lower-layer transform block andthe height newTbH of the lower-layer transform block. The decodingapparatus may perform intra prediction of the first lower-layertransform block 2621 using the intra prediction mode predModeIntra ofthe current CU and the color component cIdx of the CU. Therefore, amodified reconstructed picture of the first lower-layer transform block2621 may be generated.

For example, the decoding apparatus may generate a prediction samplematrix predSamples having a size of (newTbW)×(newTbH), by performing anintra sample prediction process. For example, the decoding apparatus mayperform an intra sample prediction process using the sample position(xTb0, yTb0), the intra prediction mode predModeIntra, the transformblock width (nTbW) newTbW, the transform block height (nTbH) newTbH, thecoding block width (nCbW) nTbW and the coding block height (nCbH) nTbHand the value of the parameter cIdx.

In addition, the decoding apparatus may generate a residual samplematrix reSamples having a size of (newTbW)×(newTbH), by performing ascaling and transform process. For example, the decoding apparatus mayperform the scaling and transform process based on the sample position(xTb0, yTb0), the value of the parameter cIdx, the transform block width(nTbW) newTbW, and the transform block height (nTbH) newTbH.

In addition, the decoding apparatus may generate a reconstructedpicture, by performing a picture reconstruction process on a colorcomponent. For example, the decoding apparatus may set a transform blockposition to (xTb0, yTb0), a transform block width (nTbW) to newTbW, seta transform block height (nTbH) to newTbH, use the value of theparameter cIdx, use the prediction sample matrix predSamples having asize of (newTbW)×(newTbH), and use a residual sample matrix reSampleshaving a size of (newTbW)×(newTbH), thereby performing a picturereconstruction process on the color component.

Next, when nTbW is greater than maxTbWidth, the decoding apparatus mayperform intra prediction on the second lower-layer transform block. Thesecond lower-layer transform block 2622 may be specified by the sampleposition (xTb0+newTbW, yTb0), the width newTbW of the lower-layertransform block and the height newTbH of the lower-layer transformblock. The decoding apparatus may perform intra prediction of the secondlower-layer transform block 2622 using the intra prediction modepredModeIntra of the current CU and the color component cIdx of thecurrent CU. Intra prediction of the second lower-layer transform block2622 may be performed on the sample position thereof similarly to intraprediction of the first lower-layer transform block 2621. Therefore, amodified reconstructed picture of the second lower-layer transform block2622 may be generated.

Next, when nTbH is greater than maxTbHeight, the decoding apparatus mayperform intra prediction on the third lower-layer transform block. Thethird lower-layer transform block 2623 may be specified by the sampleposition (xTb0, yTb0+newTbH), the width newTbW of the lower-layertransform block and the height newTbH of the lower-layer transformblock. Similarly to the above description, the decoding apparatus mayperform intra prediction using the intra prediction mode predModeIntraof the current CU and the color component cIdx of the current CU.

Next, when nTbW is greater than maxTbWidth and nTbH is greater thanmaxTbHeight, the decoding apparatus may perform intra prediction on thefourth lower-layer transform block. The fourth lower-layer transformblock 2624 may be specified by the sample position (xTb0+newTbW,yTb0+newTbH), the width newTbW of the lower-layer transform block andthe height newTbH of the lower-layer transform block. Similarly to theabove description, the decoding apparatus may perform intra predictionusing the intra prediction mode predModeIntra of the current CU and thecolor component cIdx of the current CU.

Meanwhile, the decoding apparatus may perform intra prediction asfollows, when splitting of the current transform block is not performed.For example, when nTbW is less than maxTbWidth and nTbH is less thanmaxTbHeight or ISP applies to the current CU (e.g., the value ofIntraSubpartitonSplitType is not NO_ISP_SPLIT), the current transformblock may not be split.

First, the decoding apparatus may derive parameters nW, nH, numPartsXand numPartsY as shown in the following equations.nW=IntraSubPartitionsSplitType==ISP_VER_SPLIT?nTbW/NumIntraSubPartitions:nTbWnH=IntraSubPartitionsSplitType==ISP_HOR_SPLIT?nTbH/NumIntraSubPartitions:nTbHnumPartsX=IntraSubPartitionsSplitType==ISP_VER_SPLIT?NumIntraSubPartitions:1numPartsY=IntraSubPartitionsSplitType==ISP_HOR_SPLIT?NumIntraSubPartitions:1  [Equation17]

In the above equation, IntraSubPartitionsSplitType specifies an ISPsplitting type of the current CU, and ISP_VER_SPLIT specifies verticalISP splitting, and ISP_HOR_SPLIT specifies horizontal ISP splitting.NumIntraSubPartitions specifies the number of ISP sub-partitions.

Next, the decoding apparatus may generate a prediction sample matrixpredSamples having a size of (nTbW)×(nTbH), by performing the intrasample prediction process. For example, the decoding apparatus mayperform the intra sample prediction process using the sample position(xTb0+nW*xPartIdx, yTb0+nH*yPartIdx), the intra prediction modepredModeIntra, the transform block width (nTbW) nW, the transform blockheight (nTbH) nH, the coding block width (nCbW) nTbW and the codingblock height (nCbH) nTbH and the value of the parameter cIdx. Here, thevalue of the partition index xPartIdx may have a value from 0 tonumPartX−1, and yPartIdx may have a value from 0 to numPartsY−1.

Next, the decoding apparatus may generate a residual sample matrixreSamples having a size of (nTbW)×(nTbH), by performing a scaling andtransform process. For example, the decoding apparatus may perform thescaling and transform process based on the sample position(xTbY+nW*xPartIdx, yTbY+nH*yPartIdx), the value of the parameter cIdx,the transform block width (nTbW) nW, and the transform block height(nTbH) nH.

Next, the decoding apparatus may generate a reconstructed picture, byperforming a picture reconstruction process on a color component. Forexample, the decoding apparatus may set a transform block position to(xTb0+nW*xPartIdx, yTb0+nH*yPartIdx), set a transform block width (nTbW)to nW, set a transform block height (nTbH) to nH, use the value of thepreset cIdx and use the prediction sample matrix predSamples having asize of (nTbW)×(nTbH) and the residual sample matrix reSamples having asize of (nTbW)×(nTbH), thereby performing the picture reconstructionprocess on the color component.

Encoding Method

Hereinafter, a method of performing encoding by an encoding apparatusaccording to an embodiment using the above-described method will bedescribed with reference to FIG. 29 . An encoding apparatus according toan embodiment may include a memory and at least one processor, and theat least one processor may perform the following encoding method.

First, the encoding apparatus may determine a current block by splittingan image (S2910). Next, the encoding apparatus may generate an interprediction block of the current block (S2920). Next, the encodingapparatus may generate a residual block of the current block based onthe inter prediction block (S2930). Next, the encoding apparatus mayencode inter prediction mode information of the current block (S2940).In this case, the residual block may be encoded based on a size of atransform block of the current block, and the size of the transformblock may be determined based on a color component of the current block.

More specifically, a position of a top-left sample of the transformblock may be determined based on a position and color format of atop-left sample of a luma block corresponding to the current block.

In addition, when the transform block is split into a plurality oflower-layer transform blocks, the top-left position of the lower-layertransform blocks may be determined based on the maximum width of thetransform block and the maximum height of the transform block. Forexample, the maximum width of the transform block may be determinedbased on the maximum size and color format of the transform block of theluma block corresponding to the current block, and the maximum height ofthe transform block may be determined based on the maximum size andcolor format of the transform block of the luma block corresponding tothe current block.

In addition, when the current block is a chroma block and the width ofthe transform block is greater than the maximum width of the transformblock, a plurality of lower-layer transform blocks may be generated byvertically splitting the current block. The plurality of lower-layertransform blocks may include a first lower-layer transform block and asecond lower-layer transform block, and the width of the firstlower-layer transform block is determined to be the maximum width of thetransform block, and the top-left coordinate of the second lower-layertransform block may be determined to be a value shifted from thetop-left coordinate of the first transform block to the right by themaximum width of the transform block.

In addition, when the current block is a chroma block and the height ofthe transform block is greater than the maximum height of the transformblock, a plurality of lower-layer transform blocks may be generated byhorizontally splitting the current block. The plurality of lower-layertransform blocks may include a third lower-layer transform block and afourth lower-layer transform block, and the height of the thirdlower-layer transform block may be determined to be the maximum heightof the transform block, and the top-left coordinate of the fourthlower-layer transform block may be determined to be a value shifteddownward from the top-left coordinate of the first transform block bythe maximum height of the transform block.

In addition, when the color component of the current block is a chromacomponent, the size of the transform block may be determined based on acolor format. More specifically, a width of the transform block may bedetermined based on a maximum width of the transform block, and themaximum width of the transform block may be determined based on amaximum size and color format of a transform block of a luma blockcorresponding to the current block.

In addition, when the color component of the current block is a chromacomponent, a height of the transform block may be determined based on amaximum height of the transform block, and the maximum height of thetransform block may be determined based on a maximum size and colorformat of a transform block of a luma block corresponding to the currentblock.

For example, when the color format of the current block is a formatspecifying that the width of the chroma block is half the width of thecorresponding luma block, since the maximum size of the transform blockof the luma block is 64×64, the maximum size of the transform block ofthe chroma block may be determined to be 32×64.

Decoding Method

Hereinafter, a method of performing decoding by a decoding apparatusaccording to an embodiment using the above-described method will bedescribed with reference to FIG. 30 . A decoding apparatus according toan embodiment may include a memory and at least one processor, and theat least one processor may perform the following decoding method.

First, the decoding apparatus may determine a prediction mode of acurrent block (S3010). Next, when the prediction mode of the currentblock is an inter prediction mode, the decoding apparatus may generate aprediction block of the current block based on inter prediction modeinformation (S3020). Next, the decoding apparatus may generate aresidual block of the current block based on the transform block of thecurrent block (S3030). Next, the decoding apparatus may reconstruct thecurrent block based on the prediction block and the residual block ofthe current block (S3040). In this case, the size of the transform blockmay be determined based on a color component of the current block.

A position of a top-left sample of the transform block may be determinedbased on a position and color format of a top-left sample of the lumablock corresponding to the current block. In addition, when thetransform block is split into a plurality of lower-layer transformblocks, the top-left position of the lower-layer transform blocks may bedetermined based on the maximum width of the transform block and themaximum height of the transform block. For example, the maximum width ofthe transform block may be determined based on the maximum size andcolor format of the transform block of the luma block corresponding tothe current block, and the maximum height of the transform block may bedetermined based on the maximum size and color format of the transformblock of the luma block corresponding to the current block.

In addition, when the current block is a chroma block and the width ofthe transform block is greater than the maximum width of the transformblock, a plurality of lower-layer transform blocks may be generated byvertically splitting the current block. The plurality of lower-layertransform blocks may include a first lower-layer transform block and asecond lower-layer transform block, and the width of the firstlower-layer transform block is determined to be the maximum width of thetransform block, and the top-left coordinate of the second lower-layertransform block may be determined to be a value shifted from thetop-left coordinate of the first transform block to the right by themaximum width of the transform block.

In addition, when the current block is a chroma block and the height ofthe transform block is greater than the maximum height of the transformblock, a plurality of lower-layer transform blocks may be generated byhorizontally splitting the current block. The plurality of lower-layertransform blocks may include a third lower-layer transform block and afourth lower-layer transform block, and the height of the thirdlower-layer transform block may be determined to be the maximum heightof the transform block, and the top-left coordinate of the fourthlower-layer transform block may be determined to be a value shifteddownward from the top-left coordinate of the first transform block bythe maximum height of the transform block.

In addition, when the color component of the current block is a chromacomponent, the size of the transform block may be determined based on acolor format. More specifically, a width of the transform block may bedetermined based on a maximum width of the transform block, and themaximum width of the transform block may be determined based on amaximum size and color format of a transform block of a luma blockcorresponding to the current block.

In addition, when the color component of the current block is a chromacomponent, a height of the transform block may be determined based on amaximum height of the transform block, and the maximum height of thetransform block may be determined based on a maximum size and colorformat of a transform block of a luma block corresponding to the currentblock.

For example, when the color format of the current block is a formatspecifying that the width of the chroma block is half the width of thecorresponding luma block, since the maximum size of the transform blockof the luma block is 64×64, the maximum size of the transform block ofthe chroma block may be determined to be 32×64.

Application Embodiment

While the exemplary methods of the present disclosure described aboveare represented as a series of operations for clarity of description, itis not intended to limit the order in which the steps are performed, andthe steps may be performed simultaneously or in different order asnecessary. In order to implement the method according to the presentdisclosure, the described steps may further include other steps, mayinclude remaining steps except for some of the steps, or may includeother additional steps except for some steps.

In the present disclosure, the image encoding apparatus or the imagedecoding apparatus that performs a predetermined operation (step) mayperform an operation (step) of confirming an execution condition orsituation of the corresponding operation (step). For example, if it isdescribed that predetermined operation is performed when a predeterminedcondition is satisfied, the image encoding apparatus or the imagedecoding apparatus may perform the predetermined operation afterdetermining whether the predetermined condition is satisfied.

The various embodiments of the present disclosure are not a list of allpossible combinations and are intended to describe representativeaspects of the present disclosure, and the matters described in thevarious embodiments may be applied independently or in combination oftwo or more.

Various embodiments of the present disclosure may be implemented inhardware, firmware, software, or a combination thereof. In the case ofimplementing the present disclosure by hardware, the present disclosurecan be implemented with application specific integrated circuits(ASICs), Digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), general processors, controllers, microcontrollers,microprocessors, etc.

In addition, the image decoding apparatus and the image encodingapparatus, to which the embodiments of the present disclosure areapplied, may be included in a multimedia broadcasting transmission andreception device, a mobile communication terminal, a home cinema videodevice, a digital cinema video device, a surveillance camera, a videochat device, a real time communication device such as videocommunication, a mobile streaming device, a storage medium, a camcorder,a video on demand (VoD) service providing device, an OTT video (over thetop video) device, an Internet streaming service providing device, athree-dimensional (3D) video device, a video telephony video device, amedical video device, and the like, and may be used to process videosignals or data signals. For example, the OTT video devices may includea game console, a blu-ray player, an Internet access TV, a home theatersystem, a smartphone, a tablet PC, a digital video recorder (DVR), orthe like.

FIG. 31 is a view showing a contents streaming system, to which anembodiment of the present disclosure is applicable.

As shown in FIG. 31 , the contents streaming system, to which theembodiment of the present disclosure is applied, may largely include anencoding server, a streaming server, a web server, a media storage, auser device, and a multimedia input device.

The encoding server compresses contents input from multimedia inputdevices such as a smartphone, a camera, a camcorder, etc. into digitaldata to generate a bitstream and transmits the bitstream to thestreaming server. As another example, when the multimedia input devicessuch as smartphones, cameras, camcorders, etc. directly generate abitstream, the encoding server may be omitted.

The bitstream may be generated by an image encoding method or an imageencoding apparatus, to which the embodiment of the present disclosure isapplied, and the streaming server may temporarily store the bitstream inthe process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user devicebased on a user's request through the web server, and the web serverserves as a medium for informing the user of a service. When the userrequests a desired service from the web server, the web server maydeliver it to a streaming server, and the streaming server may transmitmultimedia data to the user. In this case, the contents streaming systemmay include a separate control server. In this case, the control serverserves to control a command/response between devices in the contentsstreaming system.

The streaming server may receive contents from a media storage and/or anencoding server. For example, when the contents are received from theencoding server, the contents may be received in real time. In thiscase, in order to provide a smooth streaming service, the streamingserver may store the bitstream for a predetermined time.

Examples of the user device may include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), navigation, a slatePC, tablet PCs, ultrabooks, wearable devices (e.g., smartwatches, smartglasses, head mounted displays), digital TVs, desktops computer, digitalsignage, and the like.

Each server in the contents streaming system may be operated as adistributed server, in which case data received from each server may bedistributed.

The scope of the disclosure includes software or machine-executablecommands (e.g., an operating system, an application, firmware, aprogram, etc.) for enabling operations according to the methods ofvarious embodiments to be executed on an apparatus or a computer, anon-transitory computer-readable medium having such software or commandsstored thereon and executable on the apparatus or the computer.

INDUSTRIAL APPLICABILITY

The embodiments of the present disclosure may be used to encode ordecode an image.

The invention claimed is:
 1. An image decoding method performed by animage decoding apparatus, the image decoding method comprising:determining a prediction mode of a current block; generating aprediction block of the current block based on inter prediction modeinformation, based on the prediction mode of the current block being aninter prediction mode; generating a residual block of the current blockbased on a transform block of the current block; and reconstructing thecurrent block based on the prediction block and the residual block ofthe current block, wherein a maximum width and a maximum height of thetransform block is separately determined based on a color component ofthe current block, and wherein, based on the color component of thecurrent block being a chroma component, the maximum width and themaximum height of the transform block are determined based on a maximumsize of a luma transform block corresponding to the current block and acolor format of the current block.
 2. The image decoding method of claim1, wherein a position of a top-left sample of the transform block isdetermined based on a position of a top-left sample of the lumatransform block and the color format of the current block.
 3. The imagedecoding method of claim 1, wherein, based on the transform block beingsplit into a plurality of lower-layer transform blocks, a top-leftposition of the lower-layer transform blocks is determined based on themaximum width and the maximum height of the transform block.
 4. Theimage decoding method of claim 1, wherein the current block is a chromablock, and wherein a plurality of lower-layer transform blocks aregenerated by vertically splitting the current block, based on a width ofthe transform block being greater than the maximum width of thetransform block.
 5. The image decoding method of claim 4, wherein theplurality of lower-layer transform blocks include a first lower-layerblock and a second lower-layer transform block, wherein a width of thefirst lower-layer transform block is determined to be half the width ofthe transform block, and wherein a top-left coordinate of the secondlower-layer transform block is shifted from a top-left coordinate of thetransform block to the right by half the width of the transform block.6. The image decoding method of claim 1, wherein the current block is achroma block, and wherein a plurality of lower-layer transform blocksare generated by horizontally splitting the current block, based on aheight of the transform block being greater than the maximum height ofthe transform block.
 7. The image decoding method of claim 6, whereinthe plurality of lower-layer transform blocks include a thirdlower-layer block and a fourth lower-layer transform block, wherein aheight of the third lower-layer transform block is determined to be halfthe height of the transform block, and wherein a top-left coordinate ofthe fourth lower-layer transform block is shifted downward from atop-left coordinate of the transform block by half the height of thetransform block.
 8. The image decoding method of claim 1, wherein awidth of the transform block is determined based on the maximum width ofthe transform block.
 9. The image decoding method of claim 8, wherein aheight of the transform block is determined based on the maximum heightof the transform block.
 10. The image decoding method of claim 1,wherein, based on the color format of the current block being a formatspecifying that a width of a chroma block is half a width of acorresponding luma block, a maximum size of the transform block isdetermined to be 32×64.
 11. An image encoding method performed by animage encoding apparatus, the image encoding method comprising:determining a current block by splitting an image; generating an interprediction block of the current block; generating a residual block ofthe current block based on the inter prediction block; and encodinginter prediction mode information of the current block, wherein theresidual block is encoded based on a size of a transform block of thecurrent block, wherein a maximum width and a maximum height of thetransform block is separately determined based on a color component ofthe current block, and wherein, based on the color component of thecurrent block being a chroma component, the maximum width and themaximum height of the transform block are determined based on a maximumsize of a luma transform block corresponding to the current block and acolor format of the current block.
 12. A method of transmitting abitstream generated by an image encoding method, the image encodingmethod comprising: determining a current block by splitting an image;generating an inter prediction block of the current block; generating aresidual block of the current block based on the inter prediction block;and encoding inter prediction mode information of the current block intothe bitstream, wherein the residual block is encoded based on a size ofa transform block of the current block, wherein a maximum width and amaximum height of the transform block is separately determined based ona color component of the current block, and wherein, based on the colorcomponent of the current block being a chroma component, the maximumwidth and the maximum height of the transform block are determined basedon a maximum size of a luma transform block corresponding to the currentblock and a color format of the current block.
 13. A non-transitorycomputer-readable recording medium storing a bitstream generated by animage encoding method, the image encoding method comprising: determininga current block by splitting an image; generating an inter predictionblock of the current block; generating a residual block of the currentblock based on the inter prediction block; and encoding inter predictionmode information of the current block into the bitstream, wherein theresidual block is encoded based on a size of a transform block of thecurrent block, wherein a maximum width and a maximum height of thetransform block is separately determined based on a color component ofthe current block, and wherein, based on the color component of thecurrent block being a chroma component, the maximum width and themaximum height of the transform block are determined based on a maximumsize of a luma transform block corresponding to the current block and acolor format of the current block.